I don't think there is anything wrong with saying and showing that you can play guitar too. As a friend of mine always says, most musicians can play more than one instrument. You might be surprised. Show them you can play and they may say you should play guitar too. Talk to them about your desire to change paces.

If the one guy says he's mad and wants to leave the band all because you can play guitar too then I would consider how valuable he really is. if you can play better or even play in addition to him... and the band will sound better than that is what matters in the end.

Maybe you can arrange it so you aren't making him feel like he sucks or you are better but together playing and switching off lead/rhythm you can be better as a band...move on to a new level or a fresher sound.

I would have an open discussion and see what they are thinking...keep things open and that might limit any resentments...know what I mean? Secrets and back talk seems to be what messes up friendships and groups.

I take it you would have to look for a new drummer? Do you have any ideas of who yet or would you have to start auditioning people?

Keep us posted and good luck