You are correct, whenever you import QuickBooks Desktop company data into an Online file, the QBO company is completely overwritten with that data. It's also worth mentioning that QBO companies can only receive imported data for their first 30 days after inception. If the company is older than 30 days, please reach out to the QuickBooks Support team using the following steps:
Click QuickBooks setup & navigation > Converting to QuickBooks.
Select Get Phone Number at the bottom.
This team has special tools which can extend the date through which your company is able to import from QuickBooks Desktop. I'm also including some awesome resources to make sure you're well equipped to complete the conversion when the time comes. Our detailed guide on exporting is sure to be of help, as well as the video tutorial here:
Please touch base with me here should there be anything else you need to succeed with QuickBooks. Thanks for dropping in and best wishes.