In a piece I am writing, I want to turn an antagonist into a protagonist. I know that numerous ways to accomplish this are somewhere out there, but none of the help with a smooth transformation. Is there a way I can turn an antagonist into a protagonist in a smooth fashion?
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).
current protag is a lovebird?
Well, off the top of my head I want to say write another book from their POV. But there are several other options. My biggest idea is, in whatever method you see fit, make the current protagonist fall in love with the antagonist, now they do everything together seeming like there are two equal protagonists. Again, however you see fit, remove the original protagonist from the story. This immediately makes people think death but as you stated you want smoothness, I don't see death in any way smooth. you could use captivity, being busy, travel, or he/she might have found new love again (knowing their tendencies it was probably with the antagonist again). Hope this is helpful :)