Happened in the first movie, and of course was part of the deception in 2, but given the much improved use of de-aging special effects (Since the unfortunate X-men 3/Wolverine Origins Professor X stuff) and manipulation of older footage (Which Star Wars 9 is using, for example, for Leia) could perhaps the older Bill and Ted meet their 90's counterparts?
Carlin of course is another thing entirely though.