Originally Posted by Lt Keefer
I use monofilament line to debadge cars and trucks. It is easy and does not damage the finish. I use heat to disassemble golf clubs. The heat re-activates the epoxy holding the heads to the shafts. Heat will re-activate glues and adhesives but I don't know what it does to mirrors. Maybe a combination of the two will work on your mirror.
I would go with look at the edges first. If that's a no-go, then this might work too: a heat gun, like a paint stripper. I don't know what frames the mirror. If it's wood, you may burn it in the process, and you said you wanted to save it.
Maybe a combination: Use the gun to soften the glue without burning the border and then fishing line to make the cut into the softened glue.