Agback said:
Would you rule that the newly-ordained convert had theclass abilities of his or her total cleric level, or of a level-one cleric?
It's an interesting question which brings me back to very early days of gaming in 3ed.
First, I'll let you know that clerics changing god is something I really dislike. There are some exceptions (fallen Paladins, good clerics shifting to evil cults, evil clerics moving to a more powerful evil god), but at least good and neutral clerics should not easily forsake their religion. When they do, most of the time the character should be doomed forever and prevented from being a cleric of anyone else. In fact, these exceptions are for NPC, and how the DM deals with them it's not players' business.
Then if the rules allow cleric to switch side as easy as a politician switching to another party for personal advantage, I reserve the right to call that a "bad RP" by the player.
In your specific case, if I allowed the PC to become cleric of another god, my preference would be to restore his clerical power (with appropriate domain/spell changes if necessary) gradually.
However I also always see the powers of the cleric class as not only dependent on the god, but also on the cleric himself (the god grants them, but it's the cleric growing his inner power and levelling up*), so that it would make sense for a powerful cleric to immediately regain all his power as soon as the new god says "ok", because his level was due to himself and not the god.
*I tell my player that if it was only god's will, there would often be clerics granted higher-level spells than normal or otherwise prevented from casting spells which they should by their class level
In any case, that's my own way to do that, in case it happens it would be an exceptional situation and I can handle it freely as I see fit the campaign. If if fitted the campaign, I could strip him all class features and force him to start from level 1 as cleric of the new god, but in that case I'd probably rather apply a true "level drain" on him than mess up with having the cleric class twice. Would it be unplayable? Yes, and I'm glad it would.
But if you play by the official rules, there is only one way of doing that: one cleric class only, stripping all class features either forever or until he finds another sponsor and then back to full power.