When you switch from QuickBooks Desktop to QuickBooks Online, your past reconciliation reports don't convert to QuickBooks Online. All reconciled transactions in the register will have a reconciled status indicated with " R ".
Once everything is set, you can now start reconciling your account. After that, I'd advise you to access the Reconciliation Reportto review all the data you've previously reconciled. I'd also suggest reaching out to an accountant so they can give you the best advice on how to deal with these transactions.