I recently had the need to convert my QuickBooks Desktop file to QuickBooks Online, so I decided to try using Intuit's QuickBooks Conversion Tool to facilitate the process. Overall, I was pleasantly surprised with how straightforward the conversion process was using this tool.
The tool allows you to convert the entire chart of accounts, list data, customers, vendors, employees, and item lists from a Desktop company file into the online format. One thing I liked was that it provides an error scan before the conversion so you can correct any issues ahead of time.
The actual conversion itself was relatively fast - it took about an hour to complete uploading my 5GB company file. Everything converted smoothly, including all my historical transactions, custom fields, attached documents, etc. The only catch was that I had to reformat some reports since the online reporting differs.
But overall, I was impressed with the Conversion Tool and would recommend it if you need to convert a Desktop file to QuickBooks Online. Just be sure to back up your data beforehand! Let me know if you have any other questions.