I sent Paypal tech support an email asking this exact same question just in case I didn't get an adequate response on here. Paypal tech support emailed me saying that I couldn't update the code (per your suggestion), I'd have to recreate all of the buttons one by one in Paypal.
Huuuuh? Strange. I use that in nearly all my P.P. interfaces, in one example for the same site purchasing different services, I want the return to be a different URL. It's also how you override the "return URL" setting in the payee's account. Maybe it's just a case of context, meaning, how you are using the buttons and how it's set up.
On that, in retrospect,
If I have a button that still works, but uses their old coding, is there a snippet of code that I can add that would redirect a buyer to a specific URL of a website after checking out through Paypal?
It won't actually redirect, it refers to the "return to Merchant" link, but in the PayPal account there should be a return url setting if you only have one purchase type. Worth looking into.