Running multi-PHP versions side-by-side is possible. Getting relevant information now
Addition; while I wouldn't recommend it as a long-term replacement, a "shim" can be a very useful tool while migrating versions.
In this case, a PHP5.4 shim would allow you to continue running 5.4 code on a 7 system.
Update:
MWG does support PHP5.4! (along with 5.5, 5.6, 7.0, 7.1, 7.2, 7.3, and 7.4 - at the time of writing)