Just use this formula, whether you're changing one of the sprockets or both.
(Number of teeth on rear sprocket / Number of teeth on front sprocket) = ratio
The higher the ratio, the lower the gearing (the higher rpm at a given speed).
45/14 = 3.21
45/15 = 3.00
So the 45/15 will be geared higher, producing less rpms at a given speed
50/15 = 3.33
The 50/15 combo will be lower than what you have now, and in fact lower than your original gearing, producing more rpms at a given speed.