Have you checked that the thumbnail URL mappings are present in /Rhythmyx/rxconfig/server/addThumbnailURL.properties? (See the “Customizing Content Editor” manual for further details).
The template id may also need to be adjusted, and if you converted the thumbnail template to Velocity, its ID has changed, and the mappings will be wrong.
If that’s not to problem, please let us know. Provided that your thumbnail template is working properly, there should be no difference between “old” and “new” items.