You shouldn't enforce it at all without prior out of game consent.
Players want to build the characters they want to build. When I make a character, I make it mine. I get in touch with my perception of the character. I design their ethics based on who they are.
In general, players don't like it when you tell them their character is now someone else. Players don't like it when you take away their agency.
If your players are into this kind of thing, you won't have to enforce it.
If you are interested in implementing alignment changing mechanics, this is something you should talk about with your players. You can do this without spoiling things in the campaign.
Hey guys, I am thinking about implementing some mechanics that have the possibility of changing your characters' alignments. If this is something you are interested in or would be okay with, let's talk about how your characters are expressing their current alignment, and what changes might look like. I'm not going to spoil the campaign, but I don't want to take away your agency; so I want you to be on board with this before we get there.
This puts the power and agency into the hands of the players, even if they alignment change comes as a surprise later. If your players are on board with it, they will view it as character development that they have participated in, and you won't have to enforce anything - your players will do the work for you.