I had a custom plugin written for AE 6.5 and I am wondering if it will work in a new version of AE. It was written in c++ and Im hoping there is a way it can be compiled so it can work. If not, is there an easy way to alter / rewrite the plugin to make it compatible?
Impossible to say without any idea about the code and functions used. A ton of this stuff has changed over the years. Recompiling is of course inevitable, anyway, but outside that it will really depend on what the plug-in actually does and which API functions are called.
i also think 6.5 is 32bit, so plugins compiled for it won't run on 64bit AE versions.