Just wanted to provide an update on this.  We did upgrade from pro 2013 desktop to 2021 and we did NOT need to do the intermediate updates to our customer file.  The QB techs that helped with the installation said that the issue had been resolved in Jan 2021 and intermediate updates were no longer needed.  We had no issues with our customer file moving from 2013 to 2021.  We did have issues installing the 2021 version that our own tech had to handle, but no issues at all with the customer file converting.