The sequence of events I went through when my company changed 401k providers, because it was more confusing than it needed to be.

My company announced we were switching from Fidelity to Vanguard for our 401k. Here's what actually happened:

All existing balances transferred in-kind — my fund positions were moved to equivalent Vanguard funds. The mapping wasn't perfect: my Fidelity total market index became the Vanguard Total Stock Market Index Fund, which was fine.

Contributions and employer match continued uninterrupted through a brief transition window.

I had to re-elect my contribution percentage and fund allocation in the new Vanguard interface.

The main action items: confirm the fund mapping was correct, re-elect contribution rate, and update any beneficiary information in the new system (beneficiaries sometimes don't transfer automatically).

One thing to watch: if you had a loan against your old 401k, the transfer process can complicate things. Check with HR if that applies to you.