How to migrate a legacy 401(k) administration system.
Moving plans off an old administration system is a data project before it is a software project. A five-step method and the mistakes that cause trouble a year later.
1. Inventory what the old system really holds
A legacy administration system holds more than balances: plan provisions, eligibility history, vesting service, loans, prior testing elections, HCE and key employee history, and years of notes. List each data set, where it lives today, which fields you will need in the new system, and which you only need to archive.
2. Map fields and money sources
Build a field map from each export to the new system, including money source codes (pre-tax, Roth, catch-up, match, profit sharing, rollover), vesting schedules and date formats. Every unmapped field needs a decision: migrate, archive or drop. Write the decision down.
3. Validate before you load
Run the same validation rules you would apply to a new census: missing or impossible dates, duplicate IDs, deferrals above pay, terminated-before-hire. Fix problems in the source where possible, so the old and new systems agree. A migration that loads dirty data only moves the problem.
4. Run both systems in parallel
Process at least one full cycle in both systems for a sample of plans: payroll postings, balances, eligibility and testing. Reconcile totals by plan and by money source first, then participant by participant. Every difference gets an owner and an explanation.
5. Cut over on a date that avoids deadlines
Pick a cutover date away from the March 15 corrective-distribution and July 31 Form 5500 peaks for calendar-year plans. Freeze changes in the old system, take a final balance export, reconcile it to the new system, and keep the old system read-only for the retention period your records policy requires.
Common failure points
- Vesting service and break-in-service history not migrated, so vesting is wrong at the first distribution.
- Prior-year HCE determinations lost, so the first year's ADP/ACP testing uses the wrong groups.
- Money sources merged during mapping, so Roth and pre-tax balances cannot be separated later.
- No written acceptance criteria, so nobody can say when the migration is done.
References
- IRS 401(k) Plan Fix-It Guide
- IRS: Employee Plans Compliance Resolution System (EPCRS)
- U.S. Department of Labor, EBSA: Form 5500 series
General information for plan professionals, not legal, tax or ERISA advice. Plan documents, elections and later IRS or DOL guidance can change how a rule applies to a specific plan. Dollar figures are for the years stated; check the current year before relying on them.
Common questions
What data is hardest to migrate?
Vesting service history, prior HCE determinations and loan histories, because they are often incomplete in exports and are needed long after cutover.
Can we migrate in stages?
Yes. Migrating a group of plans at a time, each after a parallel run, limits risk.
See how KinetiPlan turns these rules into daily work.
A 15-minute live demo of the pilot build on the workflow you choose, with synthetic data.