Dont bother changed the tyre size just upgrade the stock ones to pilot sportys as Mark said, it will feel like a totally different bike especially if you have an older one with the sakura tyres.....those things were horrid or if it has sava cheapo replacements on
you want 2.75/18 front and 90/90 rear, Make sure you ask for pilot "sporty" not sport some dealers dont know they are different and say they arent available in your size