You may find that yellowed glass looks funny against fresh paint. It may look good and proper with the patina of an old paint, but why not renew the glass along with the paint? It looks bad to put old tires on new rims, or old upholstry in a newly painted cab. You can keep the old glass if you want, but you aren't saving any money by doing so. It is pretty easy to break glass putting it back in if it has started to delaminate and bubble.
Also, new glass doesn't have all the pits and grunge that old glass has on it, and is much easier to see through.
