This is a really important question.
When completing a full migration from QuickBooks Desktop to QuickBooks Online via our migration process, all data in QBO is overwritten. Unfortunately, there is no way to combine data entered in QuickBooks Online with Data imported from QuickBooks Desktop. It is very important not to begin using payroll in QBO if you plan on later importing from QuickBooks Desktop as this will create a blocker during the import process, which could include subscribing to a brand new QBO company. That error is detailed as the first error in this resource: Fix errors when you move from QuickBooks Desktop to QuickBooks Online.
Additionally, there is a 60 day limit to convert your data once you subscribe or enroll in a trial version of QBO. You can review further information on that limit here: Move your QuickBooks Desktop file to QuickBooks Online
