Lost a bit of strength / considering gear change

[DebW]

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.

An 11-21 is a seriously tight cluster that typically would be ridden only by racers or very serious high-mileage cyclists (or people who live in flat country), so don't feel bad about swapping to lower gears.

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?

The health of your knees is worth the investment. And being able to ride without worrying about running out of gears is worth the investment.

You could consider not changing the crankset but going with a wider range cassette, but that leaves you with bigger gaps between gears and would likely require a new rear derailleur. But changing to a compact may require a new front derailleur (or it may not, ask your LBS) or possibly a new rear derailleur (if it can't wrap enough chain). If cost is important, you may want to look at both options. If you really like your current closely-spaced gears, you'll prefer the compact crank with 11-23. Ask you LBS about the chainline for a compact. You may be better off with a shorter BB spindle, allowing the crankset to sit closer to the frame and giving you more usable gears on the large chainring.

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?

As long as the 2 cassettes are the same number of cogs, you should be able switch them without any adjustments. Of course, you'll want to determine that by experiment, but it should work out exactly right unless the spacing in your wheels is different.