Thank you for posting here on the Community page, FCG1.

It's my pleasure to have you here and help you out in converting a Desktop file to QuickBooks Online. 

Yes, you can start using QuickBooks Online for invoicing and estimates. When you import, your Desktop file will replace your Online file and overwrite any existing information. 

There are a few things to keep in mind when switching from QuickBooks Desktop to QuickBooks Online. You may find that transactions are missing or that reports are inconsistent.

Please refer to this article to see different information on what data don't convert when you migrate from QuickBooks Desktop to QuickBooks Online: What to expect when you switch from QuickBooks Desktop to QuickBooks Online.

Once you’re done exporting your file to QBO, run both financial reports from your QBDT and QBO to check if your accounts match. You can refer to this article for the complete steps and guidelines in moving your company file: Move your QuickBooks Desktop file to QuickBooks Online.

I’d also recommend checking our QuickBooks Support page for QBO. Here, you can browse some articles that can guide you in learning your ways around QBO.

I’ll be around if you need further help converting your file to QBO. I’d be more than happy to work with you. Have a great day ahead.