Revisions not showing for existing custom post types

Not sure if this is a bug, or an intended ‘feature’, but I created a custom type and added 20 posts to that post type, populated, made changes etc.

I have just noticed that the ‘support revisions’ box wasn’t ticked, so I have now ticked that.

However, it doesn’t appear to be creating revisions for existing posts of that post type, but works for newly created posts?

Any ideas?