Its something I wouldn't try personally. The heat
of the iron would probably cause even more damage.
I have never heard of a way to uncrease a cel.
Like it was said in the above post, if it is only
curved/bent then laying it out flat (maybe under a
book) for a while may eventually straighten them.

I have had some experience with a restoration
company. I remember they did a lot of work with
repainting cels/layers, reinking lines and that
they even deacidified paper/sketches.I don't
remember them offering to uncrease cels, but I
certainly can't remember everything they can do
off the top of my head.

Here's their website if you want to ask them.  SR
Labs It will probably be a little pricey if
they can do it, so you might also want to think
about what your limit would be on paying for a repair.