I am in the same position as the OP - I used ProSeries for 30 years, and I'm in the process of retiring and doing about a dozen paid returns for certain special clients, which makes the cost of ProSeries prohibitive (even PPR requires a minimum that is too high).  So Intuit recommended ProConnect.  It is what it is - it's not great, but I can make it work for these very simple returns. 

That being said - the data conversion process is really bad.

I followed the step by step instructions and successfully converted ten returns from 2019 ProSeries to 2020 PCTO.  It's annoying that the screen for copying the installation key has a "Next" button which does NOTHING except cycle you through the setup again - and there's nothing on that screen that says: "Now run the Data Conversion Tool directly from your start menu or desktop".  

So today I wanted to add another 2019 client to PCTO, and I again followed the step by step instructions - but I hit a dead end in the Tool, right after the screen where I tell the conversion program which software and year to convert from - because the next screen was "The following files will be sent to Intuit to be converted" - and it was just a list of the ten clients I already converted.  That's it - submit those same clients for conversion again, or go back to the "What program and year" screen - these were the only choices.

Here is what I finally figured out - and I did this by going back and telling the conversion tool that I was converting from Lacerte instead of ProSeries.  By doing that, the program allowed me to continue to the "read these instructions for backing up data files" which is the part I had forgotten about after doing the first set of conversions.  It's bizarre that these instructions are so hard to get to.  They don't exist in the online Help.  Anyway, I only had to extrapolate a little bit on the specifics, as they were LC instructions instead of PS.  Once I realized/remembered this essential step and got my new selections backed up to the ConversionData directory,  the tool added the new files to the list of converted or convertible files and I could proceed.

My review: the data conversion process is awkwardly designed and really badly executed, and the online instructions are obscure and definitely incomplete with any reference to the backup instructions.