The User Manual advices to change it while the unit is on. This is ***ONLY*** adviced as a procedure for expert technicians, not for the normal user. So it would be better and safer to do it while the unit is off and the mains cable disconnected. I believe you'll only loose your user stored info like presets, routings, setups, maps. These can be backed up and restored.
I have no full info on the battery. I believe is a a CR/BR2032 but you may want to contact support@eventide.com for precise info on this.
all the best
I
