Just guessing - install Lacerte 2020 on a local drive on a non network computer https://proconnect.intuit.com/community/download-products/help/how-to-download-next-year-s-lacerte-p...

Do the conversion there. Then upload that date to the online version. I don't use the online, so just an educated guess. 

If you are unable to do this, or it fails, you will need to contact support. https://proconnect.intuit.com/community/taxation/help/how-to-contact-lacerte-for-help/00/4691

I do know that once you do the conversion to 2020, you need to carefully review depreciation and state information for accuracy. Once you get things aligned you should go to Screen 3 and remove the check mark for Converted client (proforma use only) this is right above the break for Organizer Miscellaneous. This is for Individual only.