Welcome to the Community, @peterwm. I can provide clarification on importing data from QuickBooks Desktop (QBDT) to Online (QBO).
You can certainly import your data more than once, however, there are a couple of things to keep in mind before doing so. The first is that an import will write over and replace any data that's in the account, even that from a previous import.
Another thing to keep in mind is that while you don't have to reset anything, an account needs to have been created within 60 days for it to eligible to import. I recommend the following resource as it contains multiple articles to assist with this process, such as frequently asked questions: How to import your data to QuickBooks Online.
I believe that this information on importing will get you headed in the right direction.
Please let me know if I can be of further assistance by commenting below. Have a good one.