37 CFR 2.107 – Amendment of pleadings in an opposition proceeding
(a) Pleadings in an opposition proceeding against an application filed under section 1 or 44 of the Act may be amended in the same manner and to the same extent as in a civil action in a United States district court, except that, after the close of the time period for filing an opposition including any extension of time for filing an opposition, an opposition may not be amended to add to the goods or services opposed, or to add a joint opposer.
(b) Pleadings in an opposition proceeding against an application filed under section 66(a) of the Act may be amended in the same manner and to the same extent as in a civil action in a United States district court, except that, once filed, the opposition may not be amended to add grounds for opposition or goods or services beyond those identified in the notice of opposition, or to add a joint opposer. The grounds for opposition, the goods or services opposed, and the named opposers are limited to those identified in the ESTTA cover sheet regardless of what is contained in any attached statement.