We are in the process of implementing a new method of handling userpics when users' account types change. Currently they just keep whatever pics they had when their account was changed, but once bug 830 is resolved and goes live, there will be a new way of handling userpics.
By default, when a user has more userpics than their account type allows, their least recently used (usage is baded on their journal posts, not community posts or comments) userpics will be deactivated. This happens whenever their account type is changed or they visit editpics.bml.
If you would like to keep the current behavior for your users, simply add the following line to your ljconfig.pl:
$LJ::ALLOW_PICS_OVER_QUOTA = 1;