Yes, as Brenden said, there isn't option to downgrade after applying the hotfix. The only way at this point is to restore a backup of project before the hotfix was applied.
But when it comes to a major version... I believe that in 99.9% it really doesn't matter what exact hotfix are you running, the problem could be only between different versions of Kentico. However, of course by default it's not allowed to import web part from the newer hotfix but there is definitely no problem with importing the web part from any older hotfix of the same version.. There is just a warning displayed, but it's just an informative message.
You can ask for a download link to any older version from Support dept. or download it from the Client Portal.
Best regards, Martin