The padlock that shows up in Design Mode in the Form Editor denotes that the form is currently available for modification by users. This means the Designer enabled the "editable by users" option in the form's Property List during a previous edit of this form. If you click the padlock in the Form Editor, you are allowing change to the Source Form (by the Designer). Once you make a change and save that Source Form, all User Forms associated with that Source Form become obsolete and will no longer be usable by the current structure.
Upon creation, User Forms are stored in the
There is no other way to use the original User Forms in the current structure once the Source Form has been modified by the Designer. So you should not modify a Source Form once you've allowed users to create User Forms from it.
If you want to be able to modify a Source Form without losing access to its related existing User Forms, you can bypass the above behavior by creating a parent form that you can continue to modify, then inherit it within your Source Form. That way you can modify the parent form and never have to modify the Source Form, so that your User Forms do not become obsolete.