Before You Edit
Editing a custom form or custom field under the Setup section may have an impact on everything that the form currently exists on. One of our customers wrote in to support with the following question:
"We were under the impression that changes made to a custom form today would not affect the form on the project/task/issue that it was applied to yesterday. However, when we altered the custom form it affected all the existing items that the form was already applied to.
This is correct functionality and the expected behavior. Edits made to a custom form or custom field will impact both past and present objects that form is attached to depending on the nature of the edit.
The Work Around
A common work around for this is to make a copy of the custom form, change the information on the copy, and then use the new updated form going forward. This will allow all objects with previously attached forms to retain existing data, and future objects to have the newly edited form with updated information.