This article is from the Pratchett Newsgroups FAQ, by Orin Thomas email@example.com with numerous contributions by others.
DISCWORLD POST PTERRY.
Terry is a fairly young bloke (well my dad would say so as they were born
only 2 months apart) so is only old enough to be Uncle Pterry instead of
Grandpa Pterry so there is at least 40 years left in him yet. However, in
recent times, other author's work has been "resurrected" after their death.
This brings up the possibility, of course, of Discworld (or Johnny Maxwell
and Truckers) Novels being written by "someone else". Terry had obviously
given this some thought and has put in place the legal framework so that
NO ONE BUT TERRY PRATCHETT WRITES DISCWORLD.
Whether or not this means that a "50th Anniversary Tribute" to the Discworld
could be written for Terry whilst he is still alive in 2033 similar to what
Terry himself did for Tolkien in "After the King" (see the Bib for details)
or Asimov had this done in "Friends of the Foundation" is questionable.
(You can email Terry in 2033 to ask if you want :-)
I wonder if someone changed their name by deed pole to "Terry Pratchett"
that they would be allowed to do this :-)