Hi @ian74. Congrats on the upgrade. Although there isn't a one-click button to retrospectively sync your historical data after the upgrade, you can certainly transfer your information manually since QuickBooks Self-Employed (QBSE) and QuickBooks Online (Small Business) use different structures.
For now, I recommend downloading or exporting your tax reports to keep a copy on hand for future use. Go to the Reports menu. Use the filters to select the specific tax year or date range you need, then click the Download icon (usually a small arrow pointing down) to save the CSV file.

Next, bring your data into your new Small Business account. If you’re using the same bank or credit card from QuickBooks Self-Employed, there's no need to move transactions manually. You'll have to connect the account to QuickBooks Online, and your transactions will automatically download.
Feel welcome to reach out to me again with any questions you may have, and I would be more than happy to help.