You can open the map file, run test game, start any mode lightmapping.
When it gets past loading the objects, hold down he escape key until it returns to the editor.
This will clear lightmapping. Save the map afterwards.
Also I would make a backup of them in case something goes wrong.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.