Migration to QBO from Desktop

We will be migrating our Quick Books Desktop 2019 Premier Non-Profit to QBO.  I will do the migration quickly to see what is in the new system and prepare for hot cutover in a few months.  We need to use desktop during that time.  Will I be able to re-import data or new data to the QBO to reflect what we do in desktop.  We need to keep those books up to date.   We also have a payroll subscription in desktop and need to pay employees so when all looks good we will add payroll features online, update the books to make them current.  Is this possible?  Or does on just re-export from QB desktop and overwrite the original uploads and any testing added.  We do not want to duplicate transactions on both platforms in the test phase.