Edit or Hide Field on already Published Student Registration Form

I have a field on the student registration page that I would like to modify or at least hide.  The student form is already published and has been receiving entries, but we now want to change one of those fields.  It looks like I can't do that now that it is published?  Is this correct?  I thought that last year we were able to do that, but maybe I was mistaken about it.  Thank you for any help you can provide.