Are you saying that you had Legacy (old style) forms and you built New Style firms and now you want to enable them for use?
Or are you saying that you saying that the users are are used to using Legacy forms and you want to give them a button to view or edit on new forms?
If it's the former, you go to settings and forms and forms usage to permanently enable new forms.
If it's the later, you get the Form ID number off the list of forms and make a URL formula button like this
URLRoot() & "db/" & dbid() & "?a=dr&dfid=10&rid= & [Record ID#]
Replace the dr with er to edit the record in the new form
Replace 10 with the Form ID#