Here's my situation. Right now I have a standard double crankset and 11-21 cassette (all ultegra components on the bike) and I'm considering making the change to a compact double / 11-23 cassette.

I just starting cycling more seriously again after quite a while of un-demanding riding, and I'm noticibly weaker in the legs and knees than I was when I stopped riding seriously. Right now I find myself, much more often than I'd like, struggling with the wind and the occassional climb/wind combo in the lowest gear I have, wishing the cycling fairy would sprinkle some lower gears on my bike.

Two questions:

After a riding hiatus such as this, is it correct to think that this (hopefully temporary) change will be beneficial and worth the investment; and is there any other alternative change I should consider for this circumstance?

Second, I have an indoor trainer and an old rear wheel I plan to use on the trainer with a special tire and my 11/21 cassette. Would it be possible to interchange my 11/21 and 11/23 wheels on the fly between riding and indoor training? I'm assuming this will possible require initial derailleur adjustments and nothing else?

I plan on visiting the LBS as soon as I'm sure I want to switch to a compact double and which rear cassette, but I'd love any advise you might have before then!