Many recent studies have indicated that shorter crank lengths work fine for most people. I use a desk cycle during the winter months, and I have no trouble getting my heart rate up to the target range, despite the fact that the crank arms are about half the length of the cranks on my real bikes.

The OP is probably determined to come up with a practical way to switch effective crank lengths, but if it were me, I'd simply shorten the crank to the necessary length and leave it that way permanently.