This isn't true once you factor in things like students, band, visiting-team seating, athletic dept. tickets, recruits etc., unless you only want to have them at half the games.
UConn had 5,091 public-sale Gampel season-ticket holders last season with a total number of 8,174 categorized as season tickets, so that difference of 3,000+ fans for the venue seems to be a fixed number of the groups I mentioned above. For XL last year, those numbers were 9,083 public-sale & 10,812 total (assume the smaller difference is primarily student section) and that XL public-sale number is already way up for the upcoming season.
AD David Benedict says both arenas will be fully sold-out from season-ticket holders so even if we very conservatively say that means 11,000-12,000 XL season ticket holders plus that 3,000 fixed tickets, we are talking about thousands per game above Gampel's capacity.
And that's before things like telling fans with lower-bowl seasons at XL that their seats actually translate to upper-level at Gampel (but please keep your current donation!) and that end-zone/corner lowers are now upper corners because students have that entire side at Gampel.