Re:keeping gains from heavy cycle during a milder cycle
No would it bollocks.
I can maintain about 90% of my gains post pct, most of what you lose is water. I doubt you're at the sort of stage where you need to worry about heavy losses even if you came off altogether, let alone on 500mg sust.
Granted you lose a little bit when you come off but that's obvious considering your going from supra doses of hormones to your natural levels, or lower.
People worry too much about losing muscle though, i don't know anyone who's ran a cycle and lost all of their gains or even most of their gains, the guy i did my first cycle with still looks a lot bigger than he did and he hasnt trained in 6 months and eats whatever he likes.
Just eat at maintenance for the new mass and keep lifting heavy, in all likelihood you'll be able to gain new mass on the lower dosed cycle, it just won't be going on as quick as it would have on a heavier dose.