As long as you maintained the crossectional area, the design you are describing would almost be a transmission line listening from the end of the line.. I'm not completely sure what the effects would be, you'd probably have resonances within the chamber... Umm other then that I'm not completely sure what the effects would be. I'd say some beaming at higher frequencies, again depending on the designYou wouldn't really gain anything going from an 8" to a 12" midbass in this application. It won't necessarily sound any better, although the 12" would have more output capability because of the increased cone area and likely excursion as well. Still, a good 8" midbass, in the right box crossed at 70-80 Hz should be able to hit 105-110 db without overexerting, so it depends on your design goals...
Why do you want to switch? Maybe you could use two 8's?
Adam