I want to convert a 9.6v cordless makita circular saw to use a 14.4 battery.will the motor blow up or will it be OK and just run longer

It will not run longer, to do that you need to add more batteries in parallel not in series. It would likely burn out your motor. I have never seen anyone successfully tweek with battery packs. Maybe by tomorrow stanforty or one of the other electrical guys will jump in and prove me wrong or at least educate us both

DG agrees tried this once burnt my hand didn't work out to well.