At some point, I can imagine writing a migrator from Budgets History to the new format.
For now, unfortunately, you’d have to start over. That said, it’s easy to start a fresh Foundation spreadsheet from the Tiller Money Console. You could even use the Migration Helper workflow in Tiller Money Labs to pull over your categorization from your primary spreadsheet. From there, you’d just need to pull some budgets into the Categories sheets. You’d be ready for test in < 10 minutes.
Let me know when/if you give it a try.