I have over 472K files in LRC (11.3.1) about 400K were not the current process version (less than 5).
It took over a week of my time to upgrade the process versions. The issue was that LR would be going along andhit a file that had something wrong with it from LR's perspective and quit the process version upgrade. And all the ones that it had converted on the upgrade run stayed at their original process version. Additionally LRC did not indicate which was the bad file. This require scrolling through in grid view thousands of photos slowly becase grid view had to open the file to discover that they were bad. All in all a very cumbersome annoying process.
Desired behavior would be like Synchronize, where it continues on when there is an error, and gives a report at the end of the files that it could not convert to the current process.