So, how? How can I take my data with me as a back-up and leave? How can I migrate and upload that back-up to another instance, saving me from resubbing to a plethora of disparate communities repeatedly should I wish?
Unfortunately, there's no export/import functionality yet, that's coming soon.
Migrating does involve re-subbing to all your communities. The way I did it is I had both accounts open in 2 different windows and went down the list. Took me maybe 20 minutes.
There is a Lemmy API hosted on each instance (its how the UI works) but it's pretty technical. There is an open ticket to add an easier backup / importing feature on the lemmy github.
Additionally, it might be the sort of thing where someone could make a third party app to the hard parts for you.
Depending on how authentication works, it could be just a webapp, through which you login using your lemmy credentials, which then uses the API to collect all of your data and download it from your browser.
This question has been asked several times a day for the past week and I'd recommend using the search feature for the many answers that have already been posted.
I don't think you will be able to bring "your data" if you mean the posts since they are duplicated all across the instances. :)
But your user profile data maybe.
Actually in theory it would be possible to federate deletion of a users posts also, so it's deleted everywhere. I guess it would be like going in and clicking delete on each post manually.