Upgrading from 4.7 to 5.2.1 buildworld errors

[Ray Seals]

I have a 4.7 machine that I installed the 5.2.1 sources on (ie. copied
the /usr/src directory from a fresh installed 5.2.1 machine).  

When I do a make buildworld I get an error about "no target to make"
when it starts stage 4.1 (building includes).  

This is a production box and I don't want to build it from scratch is
there a way for me to get the 5.2.1 sources on the box and do an upgrade
that way?

Ray