I know there are like 1000 threads out there, but it's tough to find what I'm looking for.

Has anyone replaced JUST the speakers? Like with different sizing or anything? Not bypassing amps or adding crossovers or anything like that?

I just bought a 2007 G35S sedan and it sounds like the front (maybe all) speakers might need replacement just from age.

Plenty of people have done just the speakers or done speakers first as a part of a step by step system upgrade - just make sure they are 2-ohm to play well with the bose amp and they will be plug and play.

You should be able to find a thread on the speaker sizing for your year/model. I'm sure others have done more custom stuff, I just went for the easy upgrade.