This is...challenging. You see, it's possible that love changes this ex-antagonist into someone else entirely. But is that the story you're telling?
I can't tell you how to get around this, because I don't know the backstory or the story you're trying to tell. What I can tell you is little tricks I use with my own characters.
I don't like writing the 'good guy'. They are all protagonists, to be sure, but not your hero archetype, or usually even all that likeable. They curse, they cheat, they steal, they'll stab nice little old ladies in the tit for funsies. (not usually that bad, but you get the drift)
So what do I do? You make it relatable. Take a Beast and Belle (a la Beauty and the Beast) approach if you want. Let the love interest show them a new way of being. Let their shock and horror rock this ex-antagonist to their core.
Like if you take Wreck-it Ralph's approach. "You might be a bad guy, but that doesn't mean you're a bad guy."
So how would I approach it? The way I usually approach it. Give the love interest something this once bad guy would want to protect. A habit, an outlook, a characteristic that reminds them of something good and wholesome.
Alright, examples time.
- First Example: Princess Yurei.
In one story, Princess Yurei, the main character is Reiko Yanagi. Daughter to a crime boss (Japanese Yakuza), and was raised as a child soldier that was never taught empathy or basically anything remotely good. She was forced to stab her best friend in the lung when she was 4 years old. She was then raised in Nagoya (where her mother's reign of terror took place) and never made any meaningful connections because of it.
So how do you introduce a love interest that can make this piece of work into a functional human being? Well. In this world (My Hero Academia Fanfic, where there are super humans with powers called Quirks), people with Ghost-like Quirks are consistently discriminated against. Reiko has a Ghost-like Quirk, so does her love interest. Reiko, therefore, has a selfish interest in protecting said love-interest, because she's tired of people with Ghost-like Quirks being treated like criminals. And, she's so petty about it that she wants to be a hero, just to show people up--especially her long-dead mother.
- Second example: Daughters of Hyne
Another story, a Final Fantasy 8 fanfic. The main character is Luciana Lafayette, better known as Gina; she doesn't get the nickname either. She's a mercenary working for Garden (mercenary organization). There's magic, there's magical beings called Guardian Forces (or GF for short). And there are sorceresses that can access magic directly, and said sorceresses are typically hated, because of all the sorceress wars that have taken place, and over the course of the game (FF8) 2 dictators are sorceresses, and they are the mass-murdering, shoot-you-because-you-look-at-them-wrong kind.
So. Gina hates people. Her father forced her to shoot her own mother, and weeks later raped her. So yeah, Gina hates people. So how do I introduce a love interest? I use an exam in which Gina must work with them in order to pass, and show the hesitant love interest that this Gina will take on all comers to keep her safe, if she's motivated to.
Once the initial contact is made, have the love interest be willing to take a chance. It's risky as all hell for her, since she knows how people view sorceresses, but people have this weird thing about wanting to be loved. So you end up with two people with every reason not to trust others, so they understand to be very careful not to give each other reason for distrust; it won't take much for the illusion to dispel, if that becomes the case.
Add that Stella (said love interest) has a thing for healing, and she sees Gina as almost irreparably broken; challenge accepted. Add that Gina has a thing for those old movies where the dashing hero sweeps his lady off her feet (and she envisions herself in that role, sweeping her lady off her feet). And you have a gushy, gooey, heart-warming love tale, without taking either character out of character.
Gina only becomes more and more badass, because now, she knows she has to protect her lady from the world, and trust me... Gina will burn the world down if that's what it takes.
In both cases, the main character doesn't become 'a better person'. They might be selfishly motivated, they might be utter trash in human form. But they become sympathetic, and that makes for an intriguing story.
So my advice, is to give your ex-antagonist something that humanizes them. Perhaps a selfish reason to desire this love interest. Perhaps something precious they once had that they want to protect. They don't have to be 'good' to be interesting.