There’s not a hard cutoff line. Having a changing element as a part of a key, primary or not, causes work. The only question is, are you better off indexed or not. You can work out the theory for weeks, or you can try the application with and without the index.
Note also that having a changing primary key makes foreign key constraints difficult in the general case. However, either that doesn’t apply to your application, or have already solved the problem.
Linker Systems, Inc.
+1-949-552-1904 from outside North America