Just tried it on my dev site - I believe you can use an SQL query and change them, then set them hidden in the AdminCP. Just make sure you have a default style set for users that you already created and didn't edit in MySQL. But might want to ask around a little more - not quite sure how this would affect them completely.
SQL is:
Code:
UPDATE style SET styleid='thenewstyleid' WHERE styleid='theoldstyleid';
Tell me if it works alright. This would be a nice modification.