As above just take it apart and switch it all off bit by bit , I would start with switching off mains , the disconnect battery, mean while your external bell and perhaps internal bell might being going lunatic stylee , hopefully you already have your ladders setup and ready to smash it to bits if it doesn't play ball
Btw if old panel still works , leave it in place and set up your new system on the bench , set it up in how you want it to generally work and then install it after , unless your confident anyway , you can still rip out old one