Tell us more of what the plugs look like. If the plugs are just wearing out, changing the heat range won't help you. In this case, go to platinum plugs. If the plugs are getting carboned up or fouling, then switch to a hotter plug. If there is "blistering" on the insulator, then try a cooler plug. If your plug is too hot, you might also notice detonation, but that's not always a sure-thing.
One note of caution - cross referenced plugs may or may not be of the same heat range. This is an issue with many older tractors where the original plug number they specify is no longer available.