I have a trainer setup as you're describing. My spare trainer wheel does consist of wheel, cassette, & skewer (complete rear wheel unit). So it is a simple process of popping one wheel off and the other one on. The cassettes are the same in my case, 12-27, so I do not have to swap the chain. I would think that the chain for the 11-34 might be slightly long for a 12-25. I wouldn't want to swap the chain out either. But it is nice having the "rear trainer wheel" setup.
Susan
Susan
Try not. Do or do not... there is no try.
Yoda
2004 Airborne WASP (road)/Selle An-Atomica
2010 Bike Friday New World Tourist (commuter)/Selle An-Atomica
2010 Bike Friday Pocket Rocket Pro/Selle An-Atomica