V8 has the same core as v7 so most of infusions and themes should still work fine. And we no longer provide any support for v7. This version has so many security problems so it's bad idea to use it anyway. Upgrade to v8/v9 or fix it yourself.
Neverending story, v7 users and upgrade to newer version...
But you have to update php mailer also because most mailservers only accept encrypted connection V1 .2 But for that you need to upgrade your php version and then you have a conflict with php-fusion.
So for staying on php-fusion V7 you have to update it for php > 7