I currently use a spreadsheet to track my expenses, and have looked into moving to something like plaintextaccounting in the past, but I struggle to figure out how to migrate over all of my existing data. Any suggestions?
Do what the accountants do: close the old books and open new ones. Initialise relevant accounts with sums from the old books.
Edit: that said, I've found spreadsheets to be some of the most convenient ways to do high-level double entry. Just so you don't assume you need to learn a new tool to do it.