Mark
Mark wouldn't like that he hadn't been your bias, but what can you do, right? When he'd hear you tell him that a fellow member had caught your attention before him, he wouldn't be happy, but he wouldn't be jealous. He was dating you now, and the both of you were very happy and very much in love - so much so that he was sure he was your bias now. In due time it would honestly be forgotten by him completely, since he was infinitely more focus on your now than your before.
"Honestly, I kinda get it. If I was a girl, he'd be my bias too, but like, you love me now so I guess its all good."
Jaebum
Jealously, jealousy, jealousy. Absolute jealousy overload. Jaebum would not at all be happy at your admission, and maybe even get a bit moody and quiet for the whole day. It was an ego thing with him, and his had just gotten bruised. The logical part of him knew that it was whatever and not actually problematic in any way at all, but the logical part of him was tiny and insignificant compared to the jealous part. You'd have to reassure him over and over that you loved him and only him now, although he'd be sure to act extra masculine around you and your ex-bias from now on.
"Whatever, I don't care."
Jinyoung
Jinyoung would initially be none to pleased to hear this, but once he realized that your ex-bias didn't know that you had a crush on them once upon a time, he'd find it hilarious. He'd tease you all the time about it, always pretending like he'd tell his band member that you used to have a poster of them in your room just to mess with you a little. He'd have his fun with it, this one. In truth, sure it was a little hit to his ego, but it wasn't the end of the world.
"Hey Bambam, did you know jagiya right here actually used to think y-"
Jackson
Jackson would be so dramatic about it. He'd pretend to be dying when you tell him, gasping so loud you were surprised his neighbours weren't alerted. He'd clutch his heart like he was in mortal pain, looking at you with a look of open betrayal on his face that was almost enough to make you feel genuinely guilty. When he saw that you were unimpressed, he'd cut the act and get back up, pressing a kiss on your cheek.
"But I love you better than he would've, right jagiya?"
Youngjae
Sweet Youngjae would honest to god not care. He'd find it amusing and interesting and be happy to hear you talk about it, but it would do exactly nothing to impact him. To Youngjae, it would be another happy story that he got to hear that explained how you became the you he knows, and so he would take it completely in stride, the idea of being jealous or upset never once crossing his mind.
"Oh, that's so cool! What era did you like him most in?"
Bambam
Bambam would likely get a little competitive. He wouldn't be jealous perse, but he certainly wouldn't be unaffected. When you mention it, he'd laugh it off and even tease you a little bit about it, but it definitely bring out a different side of him. Whenever he sees you interacting with your ex-bias, especially if it's just you and them, he'd feel the irrational need to intervene or do something to "accidentally" interrupt his bandmate's mild flirting and your amused giggles. But of course, when confronted, he'd completely deny that he was, in fact, somewhat insecure about this situation.
"Me? Jealous? Yeah, okay, jagi, dream on."
Yugyeom
Poor Yugyeom's entire outlook would change. He would tell himself he wouldn't mind, but if he was being honest, he was bothered by it. He knew that it was a thing of the past and that the two of you had a great relationship going on. That being said, he completely understood how charming his bandmate is, and so of course it made sense that they were your bias. To make himself feel better, he'd honestly just douse you in a lot of affection, and feel even more joy than usual when you reciprocated it. With enough reassurance and physical affection, he'd get over it and go back to good old Yugyeom.
"You love me the most too, don't you, beautiful?"