as long as that Cleric aligns well to the respective faith.
Which is the crux of the problem. The deity will be well aware of how you completely didn't align with them for so long and only suddenly shifted alignment. How likely are you to remain true to their faith? Will you slip back into your old ways?
Heck, "Being a high level cleric" also means little when it's often that switching deity means starting back at level 1 (Based on prior editions of DnD this has been the case).
Which is why switching deity requires a test of faith to prove commitment to the new deity. Which is often some sort of trial or quest designed to promote the values of the deity in question.
EIther way its not the same as a fallen Paladin. Because Clerics dont have to follow a specific code and dont have to maintain a lawful good alignment.
It is very much the same. Both have some sort of guidelines to follow, if they don't they lose all their powers until they can regain them. Either through returning to ones code for Paladins or Clerics can either re-align to their Deity or opt to gain favour of a new Deity.
Just because Clerics don't have to maintain specifically lawful good alignment doesn't mean they're immune to falling. It just means that whatever alignment they must maintain is related to their chosen deity.
The only real difference is that fallen Paladins in 5e default into the Oathbreaker subclass and get to keep all their powers (Just with subclass specific ability changes) rather than losing all powers like they used to (With the "Oathbreaker" style Paladin instead being Blackguard or Antipaladin class as an alternative Paladin aligned to evil)
Anyway, all that stuff would be a hell of a lot of work to implement, thus Larian (and for that matter, previously also Bioware) opted out.
Ironically, Bioware had an easier time with it due to being able to rely on the alignment wheel (Which Owlcat does in their Pathfinder games).
Larian would have more work to do with 5e removing alignment so it would be more based off actual decisions, meaning they'd have to do Oath-like triggers for each of the dozen or so deities a Cleric has a choice for.
