It occurs to me I technically didn't add a one-time bonus for changing faiths, on that note. Remedied.
It represents increased revenues from confiscating property / eliminating the privileged status of your prior faith. Understandably you don't want to change too much.
If you're religion A and switch to B, all A provinces will pay you. If you immediately were to switch from B to C, however, you'd only receive more money from B provinces. Doing it too much will outdo the monetary benefit.
Pre-emptively stating this isn't just for Catholics. It works for ALL shared faiths. A diabolical move would be convincing an NPC to switch faiths.