Whether you got a new Stryd because your old device broke, got lost your Stryd, or you decided to upgrade to the latest version, you can keep using the same account. No need to create a different account, you can have data from many Stryds go into one account.
Each device can deviate a bit between devices and runners. So, your power can be a few % different when you get a new Stryd as compared to your older device. If you use auto-calculated Critical Power, your Critical Power may need some time to fully adjust. You can reset your CP or, eventually, your CP will be adjusted fully to your new device (maximum of three months).
If you set up your Garmin watch to pick up power from just your own Stryd by setting the ANT ID, you will need to change it. You can enter the ANT ID using Garmin Connect Mobile (go to Activities, Apps and More -> Data fields -> Settings). You can find the ANT ID on your Garmin watch.
Contact [email protected] if you need further assistance.