Great idea for a thread!
I would start right away on switching their diet.
You already know to get them fat before fasting them, so that's good.
Once they're full on feeders and are nice and chunky, I would simply fast them for two weeks, no food offering at all.
After the two week fast, try market shrimp (massivore would probably be better).
If nothing, hold back feeding another week, then try again.
I have some 7" laps I've been trying to switch onto massivore from market shrimp.
Fasted them for 3 weeks, then offered massivore and they refused. I held back another week and they couldn't help but chow down on the smelly massivore.
Now they're getting weight back on them finally; they did get pretty thin, but I've learned to not get too concerned because they're smart enough to give up after a while.