Here’s a simple question, one in which I’m not sure the PHB addresses (or, I’ve missed it, if it’s in there).
What happens when a cleric changes the god they worship?
I have a priest of the Storm Lord who has now decided to worship the Cold Rider…
I’m sure, for example, bonus feats (like that from the War domain) get lost… they can no longer cast certain spells, etc.
It’s not really like multi-classing, is it? Hmm… good question… can you worship two gods at once?!
I presume you mean “stripes”, not “strips”, in the subject. 
I will reply with the caveat that i don’t play 5E. So i don’t know if there are specific 5E rules for this. What i write below is my own take on it.
A cleric who switches deities would lose the powers that were provided. Depending on the nature of it, in particular how extreme the shift (good to evil), i think the new deity would require something of the cleric to prove their worth before full powers are granted. A quest, task, and/or rituals would need to be undertaken. Gods don’t grant their powers to just anyone, so why trust a newbie who formerly followed one of your arch enemies?
Regarding two deities at once, probably a DM’s call. I’d say yes if the deities were related, but i think there are few of those in Cerilia. Haelyn/Nesirie or Haelyn/Cuiraecen could work. In which case, i’d probably determine which powers / spells they’d receive, and only those that do not contradict one of the deities. That is- both deities would have to approve.
-Fizz
I would also suggest you consider what happens to ex-paladins.
Sorontar
Yea, I’ve always wondered why priests don’t have the same requirements as, say, druids or paladins…
It is 3e I’m dealing with here, but no worries Fizz, its roughly the same.
I’ve determined that he would keep his two granted feats, but that he has to “pay” for them with his next two level feats.
Other than that, there aren’t any other issues (aside from that which was mentioned here).
The two deities question - I think it could be possible; you pick one domain from each deity; the rest is straightforward. But, in practice, and think that clerics should be restricted to having just one deity to serve, in principle. The rules don’t seem to support otherwise…