Sounds like an easy conversion. I'd check the heat dissiaption rating on the 6 and 12V bulbs for the wattage you're looking at. In most cases, there will be more heat generated with the higher wattage bulbs but it might be marginal. Most "plastic" cases are a blend of compounds to resist not only the heat generated from the bulbs but also environmental conditions and the physical abuse of day-to-day use. See if the manufacturer uses the same material for similar wattage units it sells. As for the wire, no, it won't make much difference. The lights probably use a PVC insulated wire that will in most cases function to 105 C. If you're worried, get some Teflon insulated wire and re-wire the lamp. Anyway, good luck.