Fallen Astronauts says "Unofficially, Grissom and his crew referred to themselves as the Apollo 1 crew, and thy had that designation incorporated into their mission patch"
SP-4402, page 100 "The formula for numbering Apollo missions was altered when the three astronauts scheduled for the first manned flight lost their lives in a flash fire during launch rehearsal27 January 1967. In honor of Astronauts Virgil I. Grissom, Edward H. White II, and Roger B. Chaffee, the planned mission was given the name "Apollo 1" although it was not launched. Carrying the prelaunch designation AS-204 for the fourth launch in the Apollo Saturn IB series, the mission was officially recorded as "First manned Apollo Saturn flight-failed on ground test." Manned Spacecraft Center Deputy Director George M. Low had urged consideration of the request from the astronauts' widows that the designation "Apollo 1"-used by the astronauts publicly and included on their insignia-be retained. NASA Headquarters Office of Manned Space Flight therefore recommended the new numbering, and the NASA Project Designation Committee announced approval 3 April 1967."
Elizabeth Howell from Space.com "The first manned mission — an Earth-orbiting mission — was originally designated Apollo Saturn-204, or AS-204, but was later renamed Apollo 1."
A NASA page "The mission, originally designated Apollo 204 but commonly referred to as Apollo 1, was officially assigned the name "Apollo 1" in honor of Grissom, White, and Chaffee."
Drew Ex-Machina "While the official designation of the first manned Apollo mission was originally AS-204, early on the crew and others in the program pushed for the use of the name “Apollo 1” which was informally adopted early by many inside NASA and among the contractors. In June 1966, NASA officially approved the proposed mission patch containing the label “Apollo 1” and there was every expectation that this would become the official designation of the mission come launch which, after many delays over the course of 1966, was set for February 21, 1967 as the end of 1966 approached."
NASA SP-4009 Lots of good information.