The SWC dropped to eight teams in 1991 when Arkansas bolted from the conference. The SWC wanted to hook up with the Big 8 to form a 16-team super conference. If memory serves, the Big 8 was worried that the Texas schools would run the show and agreed to take four of the eight. We all know how that turned out.
For the record, here are schools who left from 1991 to 1996 and where the ended up:
Arkansas (1991 exit) - SEC
Baylor (1996) - Big 12
Rice (1996) - WAC, then to CUSA
SMU (1996) - WAC, then to CUSA
Texas (1996) - Big 12
Texas A&M (1996) - Big 12
Texas Tech (1996) - Big 12
TCU - (1996) WAC, then to Mountain West
Houston - CUSA.
TCU moving into the B12 South would be natural. A Texas team moving to the B10 would not be...and as discussed in other threads any school would need to be a member of the American Association of Universities. TCU is not. Neither is ND -- however I believe the conference would make an exemption for them. I doubt the same would be true of TCU.
At one time I thought TCU would make a nice addition to the B10, but after thinking about it more and looking deeper, I no loger think they would be a good fit.