Maybe some Apple Watch ex-users can give you more info but here's all I know:
Your data would stay in the Fitbit database, but there would be no connection between it and the Apple data, unless you can find some app that I don't know of to transfer the data.
Aria scale will still work but I have no idea if Apple has any way to get the weight into their database.
Maybe you could check Apple forums to see if they can get Fitbit data and Aria weights.
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