Title pretty much says it all. I have a general plan laid out for how the characters in question will grow (though one will be unrequited/have a vague conclusion and the other will end in an actual relationship), but I want some outside thoughts on this kind of change.
one word
SLOWLY
DO NOT RUSH
Give your character lots of time to move through roles. One of my characters does almost the complete opposite. He was fairly neutral, but it becomes clear he is the bad guy int he story. Think Kylo Ren, even. The more space and time your character has, the more believable the transition will be. Push them out of their role as an antagonist gently, but let them fight it a bit, let them fall back to their previous views/ways when faced with a challenge, because it is easy and familiar.
If you haven't, watch the Avatar: the last airbender. Zuko's redemption arc is possibly the greatest in all of history.
^AGREED