Can I roll Warlock skiller with a GC from pre-ROTW, that was then converted ?

Prolly not but gcs are super easy to get so I would not even try

Best thing to do is just farm a TZ for a GC, reroll that.

True true.
Its that I got like 4 or 5, 40+ lifers, so that I am sure that they are ilvl enough when rolling, kinda reassures me whenever I take a new one. No big risk or whatever but you know. Maybe also an old habit to keep all 40+ lifers from pre-TZ.

I think I'll just trash those 40+ that are still on expansion anyways and roll the ROTW ones

I dont see why not…. As rolling the GC spins the new RNG tables