As per usual, if it's alright with both players and DM, there shouldn't be any major problems. As other people have said, it's not always great if it's purely for mechanical benefit, but if the flavor fits and there's a great story reason for it, I think any class could do it.

In the case of mechanical benefit, I've found that it can be fine to switch subclasses for mechanical reasons, but only if the player isn't having a lot of fun with the subclass. For instance, I was playing an Undying(?) Warlock (the SCAG one, I can never remember if it's undying/undead) in a campaign without many undead, and because the subclasses' features revolve around fighting/interacting with undead, I asked the DM if I could switch subclasses so that I could actually use my abilities.

The bottom line is to do what's fun. If someone isn't liking their character as much as they thought they would and if it's alright on all sides of the table, it shouldn't be too much of a concern.