Ok with the heavy laser cannon, after rolling attacks you have to change all critical hits to normal.

My question is after that occurs can you then use Etahn's ability to change one of the dice back to a critical.

Yes, the HLC dice when rolled are switched from crits to hits, but other abilities may modify the dice to produce crits afterwards (so TL rerolls will produce crits and E.A. will modify a hit to a crit)

Modifiers still work to produce crits with a HLC.