I don't think there is a smooth way; an antagonist has to reverse course, there has to be a moment of truth in which they betray their followers, or partners in evil, and transition to good.
You can show doubts appearing along the way, and building to the point the antagonist hates doing what they are doing, but there will have to be a moment of truth when the antagonist flips and first offers their aid in a helpful or altruistic act; or first refuses to pursue their own interest at the expense of causing others pain; or first betrays their fellow evildoers, and that is going to be a sharp transition no matter what.
And it should be; that is a moment of significant change, and such sharp transitions make for good dramatic moments. The Bourne Identity (and whole Bourne franchise) is built on exactly this transition; a government assassin sent to kill a bad guy, that cannot bring himself to do it while the bad guy is playing with his young children.
That is told in flashback, but nevertheless it was a sharp transition, Bourne is wounded, he has amnesia, and he turns against his handlers that are then trying very hard to kill him (as a traitor).