Eh? I'm confused, =Tamar. What do you mean by "a shift into a different universe", with different rules etc.? :\ TCOM and TLF, at least, are a continuation of each other.
To me, it seems like Pterry was exploring different strands of the same universe. There's no prima facie reason why "Mort" couldn't have existed in the same universe as "Soul Music" - Mort takes place in Death's kingdom and Sto Lat. "Soul Music" takes place in A-M, Llamedos, Scrote ... and Sto Lat (does it? I forget what happens there).