The only way it's legal is if the car is only used for parades or car shows with a Historical plate on it as far as I know. Personally I would change it regardless. Not worth it.
To do a door glass on a "A" is so simple. There is a mldg on top of the door held on by I think it's 4 screws (maybe 3 it's been a long time since I've done one) There is also a mldg at the bottom of the glass opening. Take that off and simply wind the door glass out through the top (BTW if the mldgs have straight headed screws the car was built in the U.S. if they have Robertson it was built in Canada) That's what I was told by Fawcett Motor Carriage.