I converted 200 returns to ATX (from another practitioner) in the fall of 2019. Every single depreciation schedule had to be adjusted, so I would recommend converting soon so you have time to review every return.
Second what everyone said about checking carryforwards, NOLs, etc.
There were also random, inconsistent transfer issues, so just leave yourself plenty of time to open and review each 2020 return prior to the 2021 season. This was my second season with the new peeps, and it was much smoother, so when it feels unmanageable, remind yourself it is a one year issue.