Use the same account/email and no data will be lost. Your data is all stored in the Fitbit database under your account/email.
First thing is to sync from old device if, possible, to get all data from tracker to Fitbit database.
Then on phone app, tap profile pic near top left of 'Today' page to get to 'Account' page. Tap on 'Set up a Device'; select Flex 2 and it should say you already have one attached, and ask if this is a replacement - yes, and continue the process.
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