Replacement of day/night mirror glass

The glass in my rearview mirror in my 67 Barracuda is really bad. It has a day/night switch on the bottom. I can barely see anything with it. I have a similar one which I can use, but the glass is still not great, and I would like to keep the original.

I asked a local glass shop guy if he could replace the glass in it. He said he could replace it, but the day/night function would not work. I'm not sure why.

Is there somebody out there that can replace the glass in a day/night mirror and have it function as intended?

Does anybody know why the glass is special? I jut thought the switch changed the angle of the mirror.