Thanks for reaching back out to the Community, @davido.
Let's get some insight into importing your Desktop data into Online.
Since you already have 2019 information inside of QuickBooks Online, you won't be able to import that Desktop data on top of the Online data. The reason why is because it'll remove all of the 2019 information and only acknowledge the Desktop data coming into the product. However, you can always create a new account and import the Desktop data into that account so that you won't have any trouble exporting that file and losing that data.
Here's a link with some additional insight into moving your Desktop file to Online.
I want to make sure that your concerns are addressed. If you need further assistance on this matter, I'm always here. Have a wonderful rest of your week!