I'm not sure why a bigger stadium is linked so tightly in some minds with going to an FBS conference. Big, half-empty stadiums are not nearly as impressive as a jam-packed FargoDome.
Keep winning and the case for a new stadium builds itself... but it'll have to build for a long time before a tipping point is reached. Just going FBS doesn't guarantee bigger crowds - you gotta go FBS and win 8-12 games a year against interesting home opponents.