The first attempt to mass migrate failed - the user accounts came over (mostly), but none of the data from the site contents came over. So, we removed the sites and then imported a smaller number at a time. That worked.
We then found that none of the DNS settings could be moved, so we had to do this on manual.
Finally, as the "catchall" mail accounts are different on the 550 (no @www.domain), so they all had to be reset.
In our situation, with over 300 sites to move, it was a bit of a nightmare and involved an overnight shift to prep, transfer and recover. Hopefully you have a smaller number to move.