The letter comes from SAP, but the message is for the CFO: mainstream maintenance for SAP ECC 6.0 ends in 2027. Extended maintenance runs until 2030 — at a premium. After that: no patches, no security updates, no support.
For roughly 30,000 companies worldwide, this is no longer a theoretical problem. It is a countdown. And anyone who knows mid-market dynamics understands: 12-24 months of project runtime means planning must start now.
This article gives you the overview you need for a well-founded decision: Which migration path fits? What actually changes in FI and CO? What does the project cost? And which mistakes can you avoid because others have already made them?
Why now? The ECC maintenance deadline as catalyst
SAP has moved the deadline several times. 2025 was the original end. Then 2027. Now there is extended maintenance until 2030 for a 2% surcharge on license fees. Many companies treated that as a free pass — and leaned back.
That was a mistake. Because the maintenance end is not the real risk. The risk is what happens before it: SAP is investing only in S/4HANA. New features, new integrations, new industry solutions — all exclusive to the new platform. Companies that stay on ECC fall behind technologically. Quarter by quarter.
Then there is the market: consulting fees for SAP migrations have been rising by 8-12% annually since 2024. Experienced SAP FICO consultants are scarce. Companies that want to start in 2028 will compete with thousands of others for the same resources. Those who start now get to choose. Those who wait take what is left.
Brownfield, greenfield, hybrid — three paths to S/4HANA
Every SAP S/4HANA migration begins with a strategic decision: how much of the legacy system do we carry forward?
Brownfield: the system conversion
The existing ECC system is technically converted to S/4HANA. Data stays. Processes stay. Customizing stays — including all technical debt.
Pros: Faster (6-12 months), cheaper (from €250,000), less end-user training. Cons: Technical debt migrates with you. Custom code needs adaptation (SAP Readiness Check identifies affected spots). New S/4HANA features are only available after rework.
Greenfield: the reimplementation
A completely new S/4HANA system, built on best practices. No legacy, clean processes, lean customizing.
Pros: A chance to clean up. Standard processes instead of grown workarounds. Immediate access to all S/4HANA features. Cons: More expensive (€500,000 to €1.2 million), longer (12-24 months), more demanding for users. Historical data must be migrated or archived.
Selective Data Transition (hybrid / bluefield)
You selectively choose which data and processes to carry over. Master data: yes. Open documents: yes. 15 years of posting history: no. Outdated customizing: no.
Pros: Pragmatic middle ground. Cleaner than brownfield, faster than greenfield. Cons: Complex planning. Requires tool support (e.g. Syniti, SNP CrystalBridge). In our projects, about 40% of mid-market clients choose this path.
What actually changes in FI/CO
For FI/CO teams, the SAP S/4HANA migration is not an infrastructure project. It changes the data model on which your entire financial accounting and controlling are built. Three changes are fundamental:
1. ACDOCA: the Universal Journal
The table ACDOCA replaces over 20 separate FI and CO tables. BSEG, GLT0, COEP, COSS, FAGLFLEXT — all merge into a single posting table. Every posting contains all dimensions: GL account, cost center, profit center, segment, functional area.
For the general ledger: no more reconciliation between main and subledgers. For controlling: real-time access to all CO data without batch jobs. For period-end closing: 30-50% shorter cycles because reconciliation differences disappear.
2. CO-PA: account-based and costing-based merge
In classic ECC, account-based and costing-based CO-PA run in parallel — with different tables and often divergent results. In S/4HANA, account-based CO-PA becomes the standard. It runs on ACDOCA and can now handle quantities and statistical key figures too. One data model, one result.
3. Business Partner replaces customer and vendor master
The SAP Business Partner is the only master data object for business partners in S/4HANA. The old tables KNA1 and LFA1 exist only as compatibility views. Existing master data must be merged before migration — a task that is frequently underestimated and typically requires 10-15% manual rework.
Timeline and costs: what to plan for
Generic numbers help little. But after more than a dozen migration projects, we know the ranges:
Typical project timelines
- Brownfield: 6-12 months (from go-decision to go-live)
- Greenfield: 12-24 months
- Hybrid: 9-18 months
Add 3-6 months for the pre-project: SAP Readiness Check, fit-gap analysis, migration path decision, budget approval.
Typical cost ranges for mid-market companies
- Smaller installations (1 company code, minimal customizing): €250,000-400,000
- Medium installations (3-5 company codes, industry-specific customizing): €400,000-800,000
- Complex installations (international structures, SAP add-ons, interfaces): €800,000-1.2 million
License costs for S/4HANA come on top. RISE with SAP offers a subscription model starting at approximately €15,000/month, bundling license, infrastructure, and basic services. On-premise remains an option for companies with existing licenses — though with rising operating costs.
The five most common mistakes in S/4HANA migration
No migration project runs flawlessly. But some mistakes cost months and six-figure amounts. These five we see repeatedly:
1. Starting too late
"2030 is far away." By the time budget approval is secured, the partner is selected, and the project team is staffed, a year has passed. Then 12-18 months of migration. Then stabilization. Companies that start in 2028 reach go-live in 2030 at the earliest. Without buffer.
2. Underestimating master data quality
The Business Partner migration does not fail because of technology. It fails because of 50,000 customer master records with inconsistent addresses, duplicate entries, and missing fields. Master data cleansing must happen before the migration — not during.
3. Not analyzing custom code early enough
An average SAP ECC system has 2-5 million lines of custom code. Not all of it runs in S/4HANA: deprecated tables (e.g. direct BSEG access), outdated function modules, unsupported interfaces. The SAP Custom Code Analyzer identifies affected areas. Run it during the pre-project, not during implementation.
4. Not migrating to New GL beforehand
S/4HANA requires the New General Ledger. Companies still running the classic GL must make this move before or during migration. Many underestimate this — and discover it when the project plan is already set.
5. Involving the business too late
SAP S/4HANA migration is not a pure IT project. The changes in FI/CO affect every accountant and every controller. If the business side is only involved during user acceptance testing, resistance is guaranteed. Change management starts on day one.
What AI changes about the SAP S/4HANA migration
S/4HANA delivers a unified real-time data model for the first time — and with it the prerequisite for AI applications in finance. Three areas benefit immediately:
- Anomaly detection: Machine learning models analyze posting patterns and detect deviations — wrong account assignments, unusual amounts, missing dimensions. Not as a replacement for the auditor, but as upstream quality assurance.
- Cash flow forecasts: With ACDOCA data in real time, models can predict payment receipts and liquidity trends with an accuracy that manual planning cannot match.
- Automated account assignment: AI agents suggest GL accounts, cost centers, and profit centers based on historical posting patterns. Hit rates exceed 90% after a three-month learning phase.
In our migration projects, we already use AI during the testing phase: models validate migrated data against historical patterns and find inconsistencies that rule-based checks miss. On average, this saves two weeks of testing effort per migration iteration.
Conclusion: migration is mandatory. The value-add is your choice.
SAP S/4HANA migration is not optional. The only question is: do you treat it as a compliance project that causes minimal disruption? Or do you use it as the catalyst to fundamentally modernize your financial processes?
Companies that choose the second path benefit twice: a clean data model for today — and the foundation for AI applications tomorrow. The migration is the most expensive moment at which you can change the most. Use it.
We advise mid-market companies on SAP S/4HANA migration: from fit-gap analysis to go-live. With a focus on FI/CO, master data quality, and the integration of AI into the migration process. Talk to us.
