Welcome to the site !!
Dropping the front sprocket 1 tooth is equivalent to dropping 3 on the rear , so it will affect the mileage as well as your overall top speed ... What is it you are trying to achieve by dropping the gearing ?? , is the bike struggling in slower traffic situations and overworking the clutch ?? , or are you planning on trail riding more and need it to run at a slower pace overall ??
The drop will not change your mileage much if you ride at a slower pace and keep the RPM's down where they were at regular gearing , it will change , but not dramatically , however if you choose to ride at the same pace/speed with the lower gearing your RPM's will be higher at the same speed , and you will dramatically drop mileage , just like you would if you were going faster or riding more aggressively with the current gearing , if you are looking for a small difference , then I would swap the rear first , go 1 tooth at a time till you get the desired results you are looking for ..... and doing it that way , you can always compromise between the 2 extremes , instead of changing 3 teeth at once , you just go 1 , then maybe another , then maybe another till you get what you want , but this way you will be able to ride and see what each suttle change makes , because overall , 1 tooth can make a big difference , depending on your needs ....
Here is a gearing chart to help you decipher what changes are made with each sprocket change , front and/or rear , just cross the 2 gear numbers you are questioning and it will give you the overall gear ratio , you can compare that to your stock gearing ratio and decide what is best , my personal opinion is the feel , by trying 1 tooth on the rear and see what physical difference you notice good and bad , that will give you a good indication of how far you want to go , but keep in the back of your mind that 1 tooth on the front is like changing 3 on the rear