This question will undoubltly open a very stinky can of worms with a lot of folks!
While researching GM cowl tags, and just automotive accessory tags in general I found that there are two distinct, and very vocal opinions on how, or even IF someone should replace a vehicle tag. Keep in mind, I am not talking about VIN's here. It is illegal to replace those and I think that everyone is in agreement with that one. But when it come to cowl tags, that is where the room splits.
Arguements for replacement:
I would agree that if the cowl tag is very damaged or just ugly, replacing that tag on a fully resored vehicle is a nice thing to do, AS LONG AS it is the same tag with the same codes. Some might also argue that chaning the paint color or interior codes would be the interpretation of what the person wanted their car to be from the beginning. This is stepping along a thin line
Arguements for NOT replacing:
The number one arguement is one that even those who are for replacement can get behind. FAKES!
There are a number of businesses out there that will create a duplicated tag for purchase and those folks are very clear that they want very clear proof about what should be put on the tags. Others out there...well let's just say that for the right price, your 1967 Cutlass F-85 with a 6-cylinder overnight transforms into a 1967 Oldsmobile 442 W-30 and give you a VIN to match. You can read more thoughts about tag replacement by reading Brian Shook's article The Dirty Side of the Hobby located here.
The Bottom Line:
To me it all comes down to your motivation. If your intent is to recreate what was already there, then I think that is fine. Replacing your tag with something other than what was there from the factory is unethical in my opinion. Even if you are being buired in your beloved Buick GS (which is not a real GS) then I hope your air conditioning works, there is a special place for you and it is mighty hot down there.