I can definitely help you move this prior data without affecting the reconciled entries, @isofos.

Allow me to share some information first about converting Desktop data to QuickBooks Online.

When you switch from QuickBooks Desktop to QuickBooks Online, you transfer much of the information from your company file without having to re-enter it manually. Meaning, converting them will override everything and might results in duplications, missing, and unmatched entries that makes your bookkeeping inaccurate. 

So instead of the conversion process, we can manually import them depending on the type of events. Before that, export the backup data to Excel then delete the transactions that have been entered in QBO to avoid double entries. Then follow these steps to move them:

1. In your QBO account, select the Gear icon.
2. Under Tools, select Import data.
   
3. Select which type of data it is.
   
4. Click Browse and select the file. 
   

Here’s an article that has frequently asked questions about importing data to QBO. I’m sure you’ll find it helpful.

You might need this reference on how to undo reconciliation in case of some backup entries are need to be part of them. 

Don't hesitate to reach out to me here if you have further questions about importing. I’ll take care of them for you. Have a great day.