I checked Trask's sources (which isn't particularly easy since his footnotes addressed entire paragraphs, rather than sentences), and the most relevant seemed to be to the United States Army in the World War (USAWW) Series, Volume 14. So I checked that out, and while it states that "The result of this visit was an agreement that the 6 divisions were to be employed on the right of the Second Army, that the Commanding General, Second Army, was to cover the left of the French attack and that, to insure this. he should conform to the wishes of General Mangin," (USAWW 14:50) there is no mention of reflagging the Second Army. The USAWW later discusses the later formation of Third Army under Dickman for occupation duty, but again does not reference the point above.
Eventually, I did find the reference, not in Volume 14, but buried in a footnote on page 173 of Volume 1 of the USAWW, which states:
On Nov 7 Gen Pershing received a letter from Marshal Foch asking for six American divisions for a new offensive contemplated south of Metz. Marshal Foch wanted these divisions held in readiness to join in a Franco-Amerioan offensive in the direction of Château-Salins, starting from the sector east of the Moselle River. The plan was agreed to, but with the understanding that American troops should be employed under American command. Attack was to begin Nov 14 with 20 French divisions under Gen Mangin, and the 6 American divisions under Gen Bullard. Gen Pershing intended designating the American force the "Second Army." Gen Dickman was to be given the existing Second Army holding the St-Mihlel front, which would be redesignated "Third Army."
Having done this search, I was left wondering why Pershing intended to switch unit designations. My guess is that he would have been attempting to confuse the Germans and cause them to inflate US troop strength, but that's only a guess. Does anyone have any information on this?
Thanks in advance.