Moltres TM/Power Up

First order of business: congrats on the double hundos!

Secondly, I would say keep their charged moves as SA for now. SA is super effective against bug/grass, which are the two types where Moltres is an all-star. It's also more versatile for sweeping gyms (if you'd use Moltres for that). Overheat pulls ahead against steel and ice, but Moltres takes neutral damage from ice, and steel-types are often few and far between. Let BB Charizard and Entei handle ice and let Groudon and Machamp handle steel.

Plus, eventually we'll have Heatran, who may likely eclipse all the other fire types entirely. In that instance, SA may keep Moltres somewhat relevant.

Finally, Moltres isn't likely to lose OH as a charge move, so you could always TM down the road if needs be. SA, on the other hand, can't be reobtained.

I would definitly say: Don't do it!
IV's are nice to have, but they don't make that much of a difference - but Sky Attack does! In my opinion there ist no reason to downgrade a perfect Mon... I totally agree with BlameScott

Keep Sky Attack. You can't get it back and it guarantees Moltres a spot in the meta as the top DPS counter to bug and grass even if Heatran gets FS/OH and becomes the top fire type.
Congrats on your amazing catches!

I’m in a similar position where my top moltres before was a 93IV research reward (unpowered). Caught 100,98,96 IV non-shiny and 91 IV shiny. Was wondering the same thing if I should TM one or more of these to fire type.

Keep them with Sky Attack. Depending on how generous Niantic is with Heatran's move distribution SA may be the only thing to keep Moltres relevant.