Thanks for getting back in the Community, @sgwaltney. I'm here to share some details about your queries regarding data migration in QuickBooks.

Once you import your QuickBooks Desktop (QBDT) file to QuickBooks Online (QBO), it will simply copy the data and input new info to your online company. That said, when you reupload the file, the newly imported data will just be added to the current one in your QBO account. To know more, please check this article: Learn about converting from QuickBooks Desktop to QuickBooks Online. 

When ready, here's the link that you can use as a reference in performing the import process: Move your QuickBooks Desktop file to QuickBooks Online. 

Additionally, I've attached this helpful resource that you can access to get visual guides in performing your QBO tasks: Learn about QuickBooks Online with how-to videos. 

If you have any other concerns about managing your data in QuickBooks, please don't hesitate to get back here. I'm just around the corner to lend a hand. Have a good one.