Besides the safety issues others have brought up, you'd also have to replace the substations on both sides of the lines to support raising and lower at 2x the voltage. Most likely that would require roughly a 2x size in transformer and copper.

A better idea in the short term is using temperature sensors along the wire. In the winter, we should be able to push more current without risking burning out the wires if we can effectively monitor the temps in real time.