Went through almost the same thing last year and it was a mess. Moved about 800 people over on a Thursday and the sync glitch sent the welcome email three times to half the list. Got 60 unsubscribes in one day and a handful of nasty replies asking if we were broken. The worst part was the tool's own help docs told me the import was "instant," which it was not. Took their support two days to admit the tags duplicated on their end.