The X-Men franchise has gone through just as big of an evolution as its characters. But now that the MCU is taking over the lives of our favorite mutants, there's bound to be another big evolution coming.

We've already gotten a little taste of what it'll be like having the X-Men in the MCU in WandaVision, and we're excited to know which X-Men will be featured in Phase 4...or whenever they make their crossover.

But with this crossover happening in the imminent future, we also can't help wondering if they'll keep the cast from the current prequel franchise, meaning Michael Fassbender and James McAvoy and the rest of the gang, or just recast everyone. They brought over Evan Peters to play Quicksilver, so they may do it with whoever wants to come over.

The MCU has had its ups and downs recasting characters in the past. Sometimes they work really well, other times they don't. A lot of times, it can't be helped. For example, Mystique. Along with all of the older cast members in the original X-Men trilogy, Rebecca Romijn was recast so the franchise could usher in a new, younger generation of mutants. There weren't any hard feelings.

She Thinks Jennifer Lawrence Had It Easy

Romijn played Mystique in X-Men (2000), X2: X-Men United (2003), and X-Men: The Last Stand (2006). She had to endure eight hours in the makeup room for all three films as they painted her entire body luminous blue. Sometimes they brought her in at midnight to start so she'd be able to make the 9 a.m. shoot.

"You sort of have to go to a zen place," Romijn told FOX411 about the transformation process. "You have to get into some fairly awkward positions to help out the person who's painting you."

Related: Remember John Stamos And Rebecca Romijn? Facts About Their Relationship

She couldn't touch anything, and if she had to go to the toilet, well, let's just say you always knew which bowl she took.

"Blue paint on everything, including every time I had to go use the loo, blue toilet seats," Romijn recalled. "Everybody knew when I had to go use the bathroom because there was a blue toilet seat."

When it came time for Jennifer Lawrence to control the reins, there were no hard feelings between the two actresses. However, Romijn did have some words about how her successor got out of having that horrible makeup schedule. Instead of being painted for eight hours, Lawrence was lucky enough to be given a bodysuit because the makeup irritated her skin.

"Not that Jennifer Lawrence didn't do a great job; she's phenomenal. I love sharing that role with that girl. She's as cool as they come. But I do think nine hours of makeup makes you the villain you need to be to be Mystique," Romijn said.

Related: Which 'X-Men' Villain Nearly Had Their Own Solo Film?

"You can go from being perfectly happy and having a nice time to just being, like, evil bitch woman: 'If one more person stares at me, I’m gonna pull my eyes out!'" Romijn told Entertainment Weekly.

At least she had Alan Cummings, who played a similarly blue-tinted Nightcrawler, to share in the misery. "We would hang out together like an old, bitchy, blue married couple, like, 'No one understands us!'"

Besides the makeup, Romijn had prosthetics on her face that made it hard to actually act. "I don’t know if I’m getting across what I’m trying to because of all the silicone. It’s like exterior Botox without the needle. I should market it," she said.

Related: Here's What Hugh Jackman Thinks Of His 'X-Men' Co-Star, Jennifer Lawrence

At least they got rid of the contacts for the second film. "I couldn’t see, and it’s really hard to kick ass when you can’t see the ass you’re trying to kick. They did the eyes in post-production this time, but Alan had to wear his. I told him it’s a rite of passage."

There Was A Bit Of Beef About 'X-Men: Days Of Future Past'

While other female cast members were bitter that they were not brought back for X-Men: Days of Future Past, along with the other older male X-Men like Patrick Stewart and Ian McKellen, Romijn didn't really have an opinion on the matter.

Just like Romijn had no problems being booted out of the franchise when it came time for the reboot (prequels-whatever you want to call them), she didn't really have a problem not reprising her role.

"I haven’t thought about it much because they signed us to these movies three at a time, so my contract was for the first three. It’s a contractual thing.

"I don’t have an opinion on whether or not there is some kind of double standard going with regard to bringing back the older men but not, the older women," she told Entertainment Tonight. "I’d be happy to go back and reprise that role some time, but maybe they have moved on.

"Maybe they just want to keep going younger. People tweet at me all the time: 'Would you ever go back and play Mystique again?' Um, do people realize that it’s not my choice? [Laughs] Have I been asked? No, I have not been asked."

Romijn was asked back for an uncredited cameo in X-Men: First Class as an older Mystique, however. But the main reason Marvel replaced her was that the franchise wanted to explore the origins of the X-Men, and therefore, the entire cast was replaced. So it wasn't because she was bad; in fact, many fans loved her portrayal.

But Romijn thinks that Lawrence has done a fantastic job of taking the character to new heights. "I could not think of a cooler girl to share that role with. It’s not like I take issue with who has taken over the role of Mystique," she said. That's all she can really say. Her time was up, and it was time to give the character to the next person. At the rate that characters get recast in Hollywood now, it really shouldn't be taken too seriously.

Next: Without The Original X-Men Movie There Would've Been NO MCU (Here Are The Reasons Why)