It does depend on the controller. Its possible that components inside like the capacitors are rated for less than 42v, the full charge voltage of 36v.
So one possibility is that the controller will shut itself off to protect itself, as AW said. Another is that it goes pop, and you killed it hooking up 36v. The last possibility is it runs fine on 36v. Personally, I'd lean towards giving it a try. If you blow your controller, its not like you had a battery you could use on it anyway.
The motor will be fine, running double the wattage it had been. So 36v 10 amps is only 360w, no problem with the motor. I'm assuming your controller is rated to deliver 10 amps max. It might say so on it, but maybe in Chinese.
If you do end up popping the controller, look for one with 36v 15 amps, which would be in the double 250w ballpark. (36x15= 540watts)