Yes you can replace the tweeter. You just have to make sure you pick a 6.5" component set with tweeters that will fit in the factory location. That is what I did. Might take a little measuring and you'll likely have to build a bracket and/or trim the panel a little. Also yes you should be able to keep your stock head unit and run subs. You'll just need either an amp with speaker-level inputs to use instead of RCAs, or an adapter to provide you with RCA pre-outs.