Kentuckytukker and I exchanged PMs and some emails and I provided him with all the info I had on this modification. The short answer to his question is that the main changes are EL34 output tubes, a different OT to match up with them, some additional DC filtering, a different cathode resistor and bypass cap on the High Treble input triode, and some associated resistor value changes related to these mods. A bunch of the changes shown in red are associated with the addition of a variable voltage regulator, including the three 100 nf capacitor/1 Mohm resistor pairs and have nothing to do with the JTM50 conversion, per se. (As it turned out, the VVR was NOT a good addition to this amp; the one I used was a prototype I got from Keith at BYOC, and it turned out to lack enough heat dissipation to keep it from burning up after a fairly short amount of use, necessitating quite a bit of repair work.)

I'm not sure exactly when Marshall switched from a tube rectifier to solid state rectification on the JTM amps, as I've seen JTM50 schematics both ways. By the time the 50W "Plexi" came out, they were all SS rectification.