If you are trying to keep the same gear ratio and you go down on the spur you would also go down on the pinion. A smaller spur gear provides a higher gear ratio (more speed) and a bigger pinion does the same. Going from a 45 tooth spur to a 44 isn't going to make much difference and if you go down a tooth on the pinion you will be geared lower than you want. I would stick with the pinion you have.