...many priests who make so drastic a mess of things have no intention of returning. They've decided to quit, and they've simply chosen a spectacular way to announce it.
Some priests try to build a new life with a new power. It's not as hard as it sounds, really--a blood who's accomplished much for his god probably has contacts in other churches, and it's a sure bet that other churches have noticed him. They're often happy to take him in. After all, he's proven that he can draw in believers, and the defection is a blow to the old power--who's most likely a rival of the new one.
'Course, the berk's got to learn the ropes of his new faith, and he's watched carefully to make sure he's on the up-and-up. The time needed for readjustment is reflected in a loss of experience points; the exact amount lost is up to the Dungeon Master. If the new god is very similar to the old, the priest might lose only half a level. But if the two're nothing alike, the sod could lose a lot more. The DM has a good deal of leeway, but he should remember: The player character should lose no less than half an experience level, but he shouldn't drop below 1st level.
Obviously, a priest can switch gods only once or twice in his life. After the first time, he's always on thin ice with his new power, no matter how hard he works. It's tough to trust a berk who'd turn his back on faith strong enough to earn spells. Besides, some deities encourage their priests to pretend to switch beliefs, in order to sabotage another god's plans. A body'd think the powers would wise up to that peel, but they're helpless to resist. they can't ignore the lure of a mighty blood who's proven his worth by working for another god, even if they're a bit peery of his motives.