Switching of Visa type to a marriage Visa

I would ask these questions at your obshtina immigration office. I know when my friend was here on a Peace Corps visa and decided to stay and work with a different company, she had some issues getting a new visa in-country. But in the end, they allowed it because of their rule that you can apply for a visa where ever you have residence. It was just a lot of round about yes/no kinda stuff.

Since you would be using your same passport (whereas she would have to get a different passport) then perhaps you can simply extend residency based on your new status without applying for a new visa.

For example- with me, I was supposed to get permanent residence after five years of being here, through marriage. I had a son the second year I was here, and I was able to get permanent residence 3 years after my date of entry (which is what it is for parents) without changing my visa.

So yeah, sorry if that is not helpful, but I would definitely ask at your immigration office. Every office except the Varna one has always been quite helpful to me (and even sometimes in the varna one).