· Super Moderator
It's perfectly fine to change only one sprocket if you want to, especially with the low use/mileage you mention. The only issue you may have is if you change to a smaller sprocket your chain may be too long. Probably not too long if you change from 49 to 47 but you will most likely need to shorten (remove some chain links) if you change to 45. You won't know for sure until you change the sprocket and "test fit" it to make sure you have the proper length chain.