Both plastic lenses for glasses and for contact lenses could be made from a wide variety of materials. Polycarbonates (PC), polymethyl methacrylate (PMMA), thiourethanes, silicone hydrogels as well as myriads of other polymers come into play. See Wikipedia/Glasses and Wikipedia/Contact Lens. Without knowing more about the materials to be used it will be hard to answer precisely.
Some of those materials, e.g. PC and PMMA are thermoplastic or thermosoftening, thus more or less easily moldable when heated above a specific temperature. Toxic fumes due to chemical decomposition of the polymer as mentioned in the question could of course be an issue. Thermomolding therefore requires extensive air exchange (or preferably outdoor handling only) and careful heating to avoid excessive temperatures. To give an example: PMMA's melting point is around 160°C with the softening starting at even lower temperatures. Decomposing is reported to happen above 545 K (272°C) (see Thermal Decomposition of Polymers) which at least suggests that thermosoftening could be achieved without significant and hazardous decomposing.
PC and PMMA could also be "modified" using solvents such as acetone. Again be sure to take care of precautionary actions wrt to inherents hazards of those solvents (especially flammability and toxicity).
Disclaimer: I am by no means an expert in the field and therefore just reporting my opinion on the subject which should be treated as such.