It looks like to me that sorcerers can retrain bloodlines. This opens up some interesting opportunities to roleplay. "I always thought I was Celestial, but I went to the genealogy section of the archives, and it turns out I'm actually 3/64ths Draconic. As soon as I read that, I felt the blood of dragons, latent in my veins, boil to the surface!"
What's the Golarian version of 23andMe? :)
In PF1 there was a magic item, a necklace with a vial of sorcerer's blood, that let you temporarily adopt the powers of another bloodline or drink the blood to destroy the amulet but permanently change your bloodline.
I'd assume PF2 bloodline retraining is something similar, featuring a transfusion or consumption of blood from someone with the bloodline you want to adopt.
PF1's retraining rules allowed Sorcerers to retrain bloodlines also. There was even this blurb suggesting how you might flavor it:
Some of the options listed below involve retraining features of your character that are essentially permanent parts of your heritage, such as a sorcerer’s bloodline. The cost of retraining these things presumably includes magical or alchemical alterations to your body. The GM might rule that these changes are unavailable in the campaign, are only available under rare circumstances, take longer, are temporary, require some sort of quest, or are more expensive than the listed cost.
PF2 does not allow for the retraining of sorcerer bloodlines, except under the most unusual of circumstances, as dictated and permitted by the GM.
Just have to find a sorcerer with a different bloodline (and a compatible blood type) and replace all your blood with their blood. The magic will do the rest.
You could always ask a helpful incubus for advice...
Incubus: Got any demon in you?
Sorcerer: No.
Incubus: You mean "not yet."