Upgrading a 6" speaker in a combo with an 8"

How do you know that whatever 8” speaker you settle on will have a fuller response than the stock 6” speaker?
Undersized sealed cabs will tend to exacerbate the sharpness (Q) and magnitude of the speaker’s bass resonance. That’s the cause of the bass thump / honk of Marshall etc 4x12 cabs. That may not play well with a guitar amp for acoustic, as it may be prone to feedback around the bass resonance frequency.
An oversized rear mounted speaker for the baffle cut out may result in the cone hammering into the baffle.