Windows 10 Pro, with last Updates, 32 GB RAM, Intel 6 Core: Yesterday I did the update to LR C Vers 9, but before done backups. no problem with two smaller Catalog. BUT the main Catalog with about 80,000 images (Catalog-size 960 MB) had an error message "There was a problem updating the format of the Catalog file in Lightroom. This Catalog can not be used with this Lightroom version." After about 75% the upgrade will be cancelled. It was not possible to upgrade the Catalog, even with the backup-file. Also the Help.informations from Adobe wasn`t helpful. I tried it also with older backups, but still the same problem.
unfortunately I ´ve got also many troubles with my Lightroom Classic - since I got the newest Adobe updates. It crashes all the time, the making of "time lapse" videos is not possible anymore. When I start "lightroom" I get a message "catalog not found, try to chose another one". These and similary problems are identical of my two absolut different laptops ( very fast CPU, 8 GB RAM, TBs Harddisc, the newest Win10 Updates ), and they appear since newest Adobe Updates. I have already uninstalled and installed Lightroom again, but it doesn´t work!
I'm experiencing this upgrade problem also. I'm not sure if this is related but the last upgrade had "issues" like this. I finally discovered that video files were the culprit. Try to import the old catalog without video files and see if that helps.