If they are vinyl letters you could use a heat gun to soften them up and then just peel back the old stuff, no chemicals needed. Be careful not to use too much heat though, just enough to make the lettering soft and "rubbery".
No, they are one big decal (think clear bumper sticker; like a piece of packing tape with lettering on it). It is the clear part of the decal that has yellowed. There is nothing prestigous about the make and the maker is out of business anyway; I was just curious if people generally worried about that being displayed enough to replace them. I would likely put a name (undecided as yet) on instead.