Hi Michael,
You'll want to factory reset your device, which will bring the four-digit pairing code back onto the screen.
To factory reset, turn on the device screen, the scroll to Apps > Settings (gear icon) > Factory Reset?
Once you've completed the factory reset, you'll see your four-digit code on your device screen. You should now be able to sign into the Striiv app with your old account, then begin pairing the tracker to the new phone!
Michael Lee