This is what DOL told me as well, for the 2015 filing. I may try "kicking it upstairs" when I call back again.
When you call back again, take the position that any changes made to EFAST that prevent recognition of changed plan names were made in error and should be undone. The DOL cannot legitimately assert that you need to treat the plan after the name change as a separate plan for filing purposes with a new plan number. Respectfully insist that they facilitate changing the name on EFAST as before.