Thanks both for your replies. Obviously not a good idea and another one I will shelve.

I have to agree with teejay.
Heat will certainly not be good and may result in fire if excessive.
Casting usually have a gel coat (any colour) applied to the mould followed by the mat/cloth and resin. Once hardened it form a rigid and strong structure. Model hulls have frames and spacers attached to the open top to maintain the correct shape. if the mould has been stored for some time without this support it can become deformed at the open edges and gently heat can be used to help reform to the correct shape.
I suspect this is where you heard about the heating.
I would not heat much above hand heat and make sure you spread the heat all over the area to avoid local hot spots. if you are allowed a warm bath may be a suitable container.
If you make a wooden frame to the correct size this can be used to gently spread the mould to the correct shape. I suggest you then place the hull upside down on a flat building board and hold square until dry.
Cheers
dave

having used glass fibre most of my working life , I have never used heat too shape or alter the shape of any glass fibre item. how ever I have used the resin to make long shaped items (without the glass fibre ) and use boiling water to shape them to form on another shape ,such some of the decoration on post boxes ( patterns for casting)