Is it an open or closed back? If it's a closed enclosure, then you probably would want to stay away from putting in a larger driver. They build a cabinet the size they do for many reasons more than just the size of the driver.

A drivers resonance factor, frequency outputs and combing affects all come into play when they design them...

I'm not going to comment much on the open back because I don't know just how much of a factor this would have on them, however, I'm sure it would have some affect.

At least thats the way it is with loudspeaker design. Guitar amps may be a totally different ball game, but I wouldn't think so.

Having said that... It's yours man... Do what you want with it.