Try this:
1. Remove the old tracker from your account
2. Put the old tracker far away from the device you'll be syncing with (e.g. in another room), so it doesn't interfere with the new tracker
3. Perform factory reset of the new tracker. Not restart, factory reset.
4. Pair/link the new tracker with your account.
Good luck!

Marrrmaduke | Canada
Charge HR, Blaze, Charge 2, Charge 3, Inspire HR, Versa Lite, Versa SE, Aria