The plugins already loaded into memory should still hopefully be OK.

yep.

On Unix systems already loaded .so will be kept in memory. On Windows
you cannot write/replace to a file that is currently opened.

You can skip and postpone scanning of plugins that are currently in use
until the next session load.

During plugin development I check plugins usually (first) in jalv. Sometimes I forgot that I've loaded a plug already and update it, result is always a crash in jalv. 

The same happen in Mixbus4, I've just checked it, out of curiosity.