Before I updated to P3Dv4.2 I created a list of all software that had to be updated before I would install the update.
ProAtc was not on the list.
At any rate, LM leaves the legacy version of the software in my account if a situation develops where I have to uninstall any update and revert to a previous version. That has never happened yet.
Note:
On my setup I can still use ProATC in FSX or P3Dv3 or reassign the ProATC Keys as mentioned as a work around for P3Dv4.2.
ProATC has not yet released their update for P3Dv4.2.