According to instructions, Form 500 must be filed on or before the 180th day after the proposed termination date. I am not aware of a reason why the Form 500 could not be filed prior to the proposed termination date, however, it must be filed after the Notices of Plan Benefits.
(Of course the determination of who is required to receive a notice of plan benefits is based on status as of the proposed termination date, so that may be the catch.)