The B-2 designation was not "changed" to B-1a/U1. Andreas Zapf, author of "The Jet Night Fighters" which was published just last year, mentions the same meeting as you above. He states that the meeting "reveals the plans to build a 'real' Me262 night fighter, coded Messerschmitt Me262B-2, and bridge the time till this variant becomes available by building an 'interim night fighter', the Messerschmitt Me262B-1a/U1." This extract from the book seems to indicate that the meeting of September 1, 1944 mentions the interim designation.
Later in the book, he includes a copy of an original document called an "Overview of the Me262 Delivery" dated March 20, 1945. Of course, none of the new two seat night fighters is listed as delivered yet but he says that "additional sheets are available at the Bundesarchiv in Freiburg..." The sheet included does not list types by numeric designation but by description. There is a category for "Umbau" showing numbers for High Altitude PR, Nightfighter, and Two Seater versions.
One other clue is in the known Werke Numbers which seem to fall within Me262A production blocks. For instance, W.Nr. 110600 was an A-1 as was 110652. However 110636 was a B-1a/U1, the first one lost on operations. This suggests that the two seater was a conversion from an A model, hence the "U" designation.
In the front of the book, Herr Zapf invites anyone who wants to reach him to do so via email at
feedback@nachtjagd-me262.org.