So one of my players feels his character has lost faith in his god, he's currently serving Tyr in the War domain but is thinking Tempus would be better (same domain). Mechanically they're the same domain so nothing will change as far as abilities go. 

But how would you guys go about the character forsaking one god to worship another? would the PC have to prove his worth? would he keep his abilities during a period of transition? it's the first time anything like this has come up with my group.

Thanks in advance for any help! :) 

I guess this depends on how you want to work it. It sounds like since things are staying mostly the same for the stats, that this is a role-play decision. Assuming that's true, then you should discuss with the player how much RP does he want to do about it? Some players will want drama, and would love it if it took many sessions to resolve and they lost some or most of their clerical powers during the transition. They'd think it was a cool thing to RP about. Other players would hate having even 1 less spell than the book says they're supposed to at this level. Consequently, you're going to want to chat with the player about just exactly how he would like to proceed.

Assuming the player wants to do substantial RP about it, then, depending on how you interpret Tyr and how vengeful Tyr is, there might be some long-lasting problems with leaving a deity. Tyr might send servants to punish the character. Perhaps there would be some enmity between Tyr and Tempus over this, if it's a significant enough cleric (high enough level -- probably not if he's level 2). But I wouldn't try doing this unless the player wants to RP about it.

One question I think Tempus would naturally have to ask is: how do I know you won't abandon me for the next god after I give you all that you want from me?

Maybe I'm making this too easy but I ran into this situation with a player who violated his clerical oath to his diety in the life domain and decided it best how he wanted to play his character to change to another domain / diety.  I ended up taking away his spell with the exception cantrips and he then had to devote to his new diety for at least a short adventure or how well the player RP the situation.  The goal was to have the character struggle without his base spells but also praying to the new diety to gain favor.  Now I gave credit for RP the situation because the player felt he wanted to earn the change and not just handed it over with open arms.   

Yes, it hurt the player short term as a cleric but it's working with the party and a (beep) ton of Potions of Healing.

I think that is a good way to work it (taking away the spells and letting him earn them back). However, some players will feel they are being punished if you do that, so the key is to speak to the player. Some players love the RP of it and some don't.