You can't really back up a note and migrate them to another account.
All messages and notes are signed by your private key. Another account would have another private key. What you're asking is to resign all notes and republish them as if they were made by someone else.
1. Some relays will refuse to accept notes back dated so far in the past.
2. Every note has an ID but that ID is auto-generated based on the message, the author and so on. You can't make another note with the same ID.
3. A note with a different ID won't carry with it the replies, likes and references that the old note had.
It would be cool to understand your use case as there may be a way to design something that suits your use case while still following the rules of nostr.