Not that I've ever heard of. There were extensions to the PDP-8 for a few things (multiply/divide, extended memory addressing,) but never a full architectural upgrade, because DEC had other computer lines for people who wanted something with more oomph to it, and the -8's main appeal was that it was a simple, small, low-cost minicomputer for people who didn't need something fancier.
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
Synthesizers: Roland JX-10/SH-09/HS-80/MT-32/D-50, Yamaha DX7-II/V50/TX7/TG33/FB-01, Korg MS-20 Mini/ARP Odyssey/DW-8000/M1, Ensoniq SQ-80, E-mu Emax HD/Proteus-2, Casio CZ-5000, Moog Satellite, Sequential Circuits Prophet-600
"'Legacy code' often differs from its suggested alternative by actually working and scaling." - Bjarne Stroustrup