Microsoft® SharePoint® Designer 2010 Unleashed

Best Practices for Customizing List Forms in SharePoint 2010

In SharePoint Designer 2007, the recommendation was to not delete the original list forms, such as NewForm.aspx, DisplayForm.aspx, or EditForm.aspx, but instead create new forms and hide the existing forms. In SharePoint 2010, this is still the case. For instance, when you choose to create a custom list form via the Forms part on a list settings page, a new ASPX page and form is created, which you can then nominate as the default form type for the list.

One reason for not deleting original forms is that if you delete a custom form that is currently selected as the default new, edit, or display form for a list, then SharePoint automatically promotes an equivalent form type as the default ...