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Hello @ahegoukian, thanks for joining the Fitbit Community, it's great to have you on board. 
I appreciate your participation in the Forums and for sharing your experience with us. I would also like to thank you for letting us know the troubleshooting steps you've tried so far. Please note that when switching phones, you don't need to remove the tracker from your account in order to set it up again, simply open the Fitbit app on your new phone and sync your device. With that said, it's possible your Alta HR still has a Bluetooth connection with your old phone, please go to the Bluetooth settings on your old phone and check if your tracker is on the list of paired devices. If it is, tap on it and select Forget this Device.
After that, try to set up your Alta HR to your account again:
- Open the Fitbit app and tap on the Account symbol at the top right corner of the screen, then tap on "Set up a New Device"
- Choose your tracker and follow the onscreen instructions to continue
- When the app asks you to replace your current tracker, please replace it and continue with the process.
- When a pop-up appears asking you to Pair your tracker to your device, tap on Pair and continue.
I hope this can be helpful, give it a try and let us know the outcome.
Best Answer
