37 CFR §2.73

An application that includes section 1(a) of the Trademark Act as a filing basis, or for which an acceptable allegation of use under § 2.76 or § 2.88 has been filed, may be amended to an application for concurrent use registration, provided that the application as amended meets the requirements of § 2.42. The trademark examining attorney will determine whether the application, as amended, is acceptable.