I think it’s lame the move to 5* is a one way street. I get the idea, you make your choice, but in the era of s class, I think you should be able to revert back to 4*.
For regular weapons this isn’t that big a deal, but with character weapons, it prevents you from taking a toon in a different direction down the road. Say you get some new characters, the meta changes, whatever, you want to try a new team. If you locked in character weapons at 5*, you may be sunk when developing a new strategy.