Ok first off, I do not believe users will be switching back and forth from analog <-> digital. They might at first just to see the two, but then they will pick one and stick to it.
Second, if you have a 24 hr digital clock selected and go to analog, it will pick (if it exists) the 24 hr variant of the theme. You can still change this to the 12 hr variant!
Given those 2 facts, there seems to be no need to do anything at all.
Ok first off, I do not believe users will be switching back and forth from analog <-> digital. They might at first just to see the two, but then they will pick one and stick to it.
Second, if you have a 24 hr digital clock selected and go to analog, it will pick (if it exists) the 24 hr variant of the theme. You can still change this to the 12 hr variant!
Given those 2 facts, there seems to be no need to do anything at all.