To start with the rating on motors doesn't mean as much as you might think. For example a 500w motor running with a 35a controller will actually produce more then twice it's rating. Most controllers are rated in amperage but some distributors like to use watts. For sure you will have more torque and speed using a 1200w controller over a 500w controller. Will it damage the motor? Not likely but I would check on the temp. of the motor after a hard run. It should never get so hot that you don't want to hold you hand on it. If this does happen just don't use full throttle except when you need added power for climbing and or exceloration. The rest of the time use a reduced throttle and you should be fine. Using over size controllers in this manner is very common. Keep in mind that you need a battery pack that can handle the 1200w controller too.