37 CFR § 2.75 - Amendment to change application to different register.
(a) An application for registration on the Principal Register under section 1(a) or 44 of the Act may be changed to an application for registration on the Supplemental Register and vice versa by amending the application to comply with the rules relating to the appropriate register, as the case may be.
(b) An application under section 1(b) of the Act may be amended to change the application to a different register only after submission of an acceptable amendment to allege use under § 2.76 or statement of use under § 2.88. When such an application is changed from the Principal Register to the Supplemental Register, the effective filing date of the application is the date of the filing of the allegation of use under section 1(c) or 1(d) of the Act.
(c) In an application under section 66(a) of the Act, the applicant may not amend the application to the Supplemental Register.