In PoA, when Lupin forgets to take his potion, he doesn't actually change until they go outside and the moon comes out from behind a cloud

Please don't remind me. I try to repress that passage whenever possible, because it makes no bloody sense.

If the potion stopped the change altogether, then maybe we'd have wiggle room (i.e., "Oh, well he took it for six days, so the change was delayed until he was actually touched by moonlight"), but no!