I'm not sure if you still need this answered, but you could absolutely do this. The only thing is that you may be going 1 mph faster than your speedometer indicates. The reason is because you are increasing the diameter of your tire. There is no danger though.
Even then that won't be an issue, but it's good you brought it up, better than him thinking something is wrong and not even considering it's the tires.
Better get those tires on now!