Plugins and other extensions should be backwards compatible yes.. but you should *test* this. We've run into some issues where our plugins were not working properly and have had to make adjustments to the minPHP bridge. As Mike mentioned, you may want to update your buttons in your plugins to use the new Bootstrap ones, but if you don't they should still work.
Test, test, test, everything you can think of. It's possible we missed something, and being a major release, this version requires extra scrutiny.