Q: Do i have to make a resync?
A: Short answer: No. Long answer: Read on:
No, that is not mandatory. However, we recommend doing so to prevent losing incoming mails. Lets say you do a migration, then you switch you provider or domain. Meanwhile there is a possibility that emails are still coming in. After switching, a resync can transfer those messages to the new account, so no message gets lost.
We strongly recommend doing a resync some days after the initial migration (After switching your DNS settings if you plan so)