Reclassing npc's

Hi everyone. I've been playing the Baldur's Gate and Icewind Dale series since they've come out and followed them since Bioware had little tombs of them on a bookshelf on their website to click on years ago.

I've partied up with everyone hero and villain in the, or near to it, and recently I made a party to incorporate all the new heroes. I love the voice work of the new heroes though I have trouble sometimes keeping Rasaad alive.

So I've put some thought into it and drew up my own 6 member party of self made heroes. Long as you can still do this I was going to do it through multi-player and save it over to my single player saves.

I'd rather reclass some npc's though, I did that years ago through Shadow Keeper. Problem was getting their stats correct, like their save vs throws, to be correct for their new class. If I take Mincs for example and give him 1 experience point and change him to level one cleric (for example) and adjust his save vs throw and stats, will he level up the same way a cleric would or would his programming only level him up like a ranger?

Thanks.