401(k) provider migration — what's the participant communication look like in practice?

We're moving from Empower to Fidelity for our 220-EE 401(k). The fund mapping is sorted but the blackout-period communication to participants is keeping me up at night.

For anyone who's run this transition: what was the actual participant complaint volume during the blackout? Did anyone successfully reduce friction with proactive comms, or is it just an unavoidable 4-week annoyance? Any compliance surprises (404a-5 disclosures, blackout notices)?