Cleric switching deities...

[atemu1234]

If a cleric switches his deity, does he change his domains? Moreover, can he switch his deity in the first place?

[holywhippet]

I'd list this under DM fiat to be honest. You can technically be a cleric without worshipping any given deity. You could worship a pantheon or even just have a general principle your faith is based on without needing a deity. But I can't think of any rule that allows a cleric to change domains or deities off the top of my head. You could maybe justify it via roleplaying - like if you lost faith in your previous belief system because of some traumatic event. The rules allow for ex-clerics if they break their code of conduct, but that only loses you class features and atoning will restore you to your previous faith.

[qwertyu63]

If a cleric switches his deity, does he change his domains? Moreover, can he switch his deity in the first place?

Yes and yes; see Player's Handbook II, page 193.