Ken --

Rucker's rank did not change, only his assignment. Since the date the 6th Cavalry Brigade was dissolved (July 18th) and the date Rucker received a new assignment (Aug. 30th) come directly from the O.R., we can be confident about that part. Incidentally, Rucker's command of the 6th Cavalry Brigade dates from its creation on May 24, 1864.

References are found as follows --

Volume XXXIX, part 2, page 619:
GENERAL ORDERS No. 47, HEADQUARTERS FORRESTS CAVALRY, Tupelo, May 24, 1864.

I. The Seventh Tennessee Regiment, Colonel Dnckworth, the Nineteenth Mississippi Regiment, Colonel Duff, and the Eighteenth Mississippi Battalion, Lieutenant-Colonel Chalmers, will hereafter be known as the Sixth Brigade, and will report for duty to Col. E. W. Rucker, who will assume command of it. Colonel Rucker will act under verbal instructions fromn these headquarters.
II. Capt. J. C. Thrall, with his battery, will immediately report for duty to Colonel Rucker, to whose brigade it is assigned.

By order of Major-General Forrest: W. H. BRAND, Acting Assistant Adjutant-General.

Volume XXXIX, part 2, pages 717-18:
SPECIAL ORDERS, No. 106, HEADQUARTERS FORRESTS CAVALRY, Okolona, July 18, 1864.

The command heretofore known as the Sixth Brigade, commanded by Col. E. W. Rucker, is dissolved. The Eighteenth and Nineteenth Mississippi Battalions will be returned to the brigade commanded by Col. R. McCulloch, and the Seventh Tennessee will be assigned for duty to Neely's brigade. The regiment now known as Forrest's regiment, commanded by Lient. Col. D. C. Kelley, will be assigned to duty with Neely's brigade. Each one of these regiments will retain its present transportation. The ordnance and headquarters wagons and ambulances of the Sixth Brigade will be turned over to the chief quartermaster of Chalmers division.

By command of Major-General Forrest: W. H. BRAND, Acting Assistant Adjutant-General.

Volume XXXIX, part 2, pages 805-06:
GENERAL ORDERS No. 73, HEADQUARTERS FORRESTS CAVALRY, Grenada, August 30, 1864.

I. The troops of this command will be organized and designated as follows: Rucker's brigade, Col. Edmund W. Rucker: Seventh Tennessee Cavalry, Col. William L. Duckworth; Neely's regiment Tennessee cavalry, Col. James J. Neely; Twelfth Tennessee Cavalry, Col. Robert V. Richardson; Stewarts regiment Tennessee cavalry, Col. Francis M. Stewart, Twenty-sixth Tennessee Battalion Cavalry (General Forrests old regiment) Lieut Col. David C. Kelley.
II. Col. E. W. Rucker is assigned permanently to the command of the brigade designated as Rucker's brigade.

By command of Major-General Forrest: J. P. STRANGE, Assistant Adjutant-General.

Volume XXXIX, part 2, pages 831-32:
HDQRS. CHALMERS DIVISION FORRESTS CAVALRY, West Point, Miss., September 12, 1864.
Col. F. M. STEWART, Fifteenth Tennessee Cavalry; Col. J. U. GREEN, Twelfth Tennessee Cavalry; Col. W. L. DUCKWORTH, Seventh Tennessee Cavalry; Col. J. J. NEELY, Fourteenth Tennessee Cavalry; Maj. P. T. ALLIN, Twenty-sixth [Tennessee] Battalion:

GENTLEMEN: I am directed by Brigadier-General Chalmers to say to you, in reply to your note of this morning, that by paragraph I of General Orders, No. 73, from Headquarters Forrests Cavalry, dated Grenada, August 30, 1864, the regiments under your respective commands were organized into a brigade, to be designated as Ruckers brigade, and by paragraph II of the same order Col. E. W. Rucker was assigned permanently to the command of that brigade. Colonel Rucker having reported for duty, you will obey promptly all orders issued by or coming through him. A copy of the order referred to above was sent some days since from these headquarters to Colonel Neely, who was them commanding your brigade, and it was his duty to have issued copies of it promptly to the different regiments under his command, but the general learns with regret that this duty was neglected, and the order permitted to lie unpublished in his office, a neglect of duty which deserves censure. As, however, Colonel Neely had regular official information of the existence of that order, which was, indeed, in his own hands, and as some others of you had verbal information of the existence of such an order, the general thinks that the fact that you have not received official copies of it affords scarcely a decent pretext for hesitating to obey orders issued by or coming through Colonel Rucker.
I am, gentlemen, very respectfully, your obedient servant,
W. A. GOODMAN, Assistant Adjutant-General.