Amended income tax returns reflecting any increase or decrease in tax attributable to the election shall be filed for the taxable years to which the election applies. In the case of partnerships and electing small business corporations under subchapter S, chapter 1 of the Code, amended returns shall be filed by the partnership or electing small business corporations, as well as by the partners or shareholders, as the case may be. Any amended return shall be filed with the office of the district director with whom the taxpayer files his income tax return for the taxable year in which the election is made and, if practicable, on the same date the statement of election is filed, but amended returns shall be filed in no event later than February 28, 1961, unless an extension of time is granted under section 6081 of the Code. Whenever the amended returns do not accompany the statement of election, a copy of the statement shall be submitted with the amended returns. The amended returns shall be accompanied by payment of the additional tax (together with interest thereon) resulting from the election.
26 C.F.R. §1.9003-4