You should tiptoe in before you jump in headlong. Just don’t run presence from old and newapp at the same time and don’t run STM/STHM (Classic and newapp security module) in the same location at the same time
You can absolutely run both at the same time and see what looks ‘broken’ in the new app. What most have seen, issues could range from poorly displaying device handlers (custom ones if you use them) to how your routines would get transferred into automations and Scenes, but at least by stepping through each of these manually you control your destiny on the fix rather than having it all happen at once