Changing this define will make your plugin appear as a different plugin to the Host, so this will be a session breaking change. I remember having a hard time figuring out why a plugin wasn’t recognized anymore after porting it from a Projucer project to CMake a while ago and in the end I found out that it was because of both projects defining JUCE_VST3_CAN_REPLACE_VST2 different. This topic is about exactly that 
So, better don’t do this