I have a fork of Discourse that I use when I want to hack Core.
I use it for experimenting only and I use it accepting the fact that with every change I make I risk totally trashing the fork (I have done so numerous times) and that I will need to start over with a fresh install (also done numerous times).
Nothing personal, but as you are unsure what “fresh install” means I have a strong feeling you should not hack any Core yourself. It is much less stressful to hack, break, and fix a plugin than it is to try to restore Discourse Core to working order after it has been broken.