Also it's interesting to note that Beauregard wouldn't be available to do his historical Bermuda Hundred Campaign. Perhaps Johnston would be assigned there. Who knows if he could replicate Beauregard's success.
If Beauregard assumes command of the Army of Tennessee as early as December 1863 (aka when Bragg resigned from command), Johnston might have been let at the head of the Department of the West, controlling technically both Army of Mississippi under Leonidas Polk and Army of Tennessee under Beauregard. The defenses of Petersburg, North Carolina and Southern Virginia can be assumed directly by Braxton Bragg, under the supervision of Davis. Bragg is mostly at his best on training and organizing troops and could therefore manage his command as efficiently as did Beauregard (despite his tactical flaws).
Johnston being so closed of Richmond (and Davis) would be kind of a challenge for Davis. The further he is, the better it is for the harmony of military operations.
So, Johnston can remain in command of his Western Department, leaving Beauregard in charge of tactical operations while Johnston himself is taking charge of both strategic and logistic actions. Johnston is therefore able to move troops from an army to another (Polk's Army of Mississippi, Maury's Army of the Gulf and Beauregard's Army of Tennessee).
But I assume your question was :
Could Beauregard perform as well as Johnston at the head of the Army of Tennessee during the Atlanta Campaign ?
And my answer is :
Yes, I believe so (at some point).
When Beauregard was resisting against the Union advance towards Corinth during April-May 1862, he handled his army quite well and managed to evacuate his whole command before being assaulted. He shared some of Johnston's defensive traits, especially concerning the mobility of an army in campaign. He may had an offensive spirit at the tactical level but he knew when and where to strike and when and where to flee, considering his chances of success based on information he gathered (for example, sending cavalry units scouting enemy lines and rears). So, this general has
potential, and he'll never be worse than Bragg at the head of this army. His popularity, more than his organizational skills, would be his greatest advantage to improve the morale of disheartened soldiers.
One more point : Johnston wanted to change the command structure of the Army of Tennessee as early as January 1864 (O.R. S1-Vol.32, pp. 564-565), before the Atlanta Campaign. He had the First Corps under LTG Hardee and the Second Corps under MG Hindman. Hardee's command had four divisions (Cheatham's, Stevenson's, Cleburne's and Walker's) while Hindman's command was composed of three divisions (Hindman's, Breckinridge's and Stewart's). Johnston asked Cooper several times to create a Third Corps in order to improve the maneuverability of the army, with LTG Hardee commanding three divisions, MG Lovell commanding two divisions and MG Hindman commanding two divisions. I believe Beauregard would organize the army this way, creating three flexible commands rather than keeping two huge formations (Mansfield Lovell being too much criticized for the loss of New Orleans, another general can therefore take his place, either MG Benjamin F. Cheatham, MG Carter L. Stevenson or MG W.H.T. Walker).
*Army of Tennessee : Gen. P.G.T. Beauregard
- First Corps : Lt-Gen. William J. Hardee
- - - Stevenson's Div. : Maj-Gen. Carter L. Stevenson
- - - Cleburne's Div. : Maj-Gen. Patrick R. Cleburne
- - - Walker's Div. : Maj-Gen. W.H.T. Walker
- Second Corps : Maj-Gen. Mansfield Lovell / Maj-Gen. Benjamin F. Cheatham
- - - Cheatham's Div. : Maj-Gen. Benjamin F. Cheatham / Brig-Gen. George E. Maney
- - - Breckinridge's Div. : Brig-Gen. William B. Bate
- Third Corps : Maj-Gen. Thomas C. Hindman
- - - Hindman's Div. : Brig-Gen. John C. Brown
- - - Stewart's Div. : Maj-Gen. Alexander P. Stewart
A nice "What If", leading to much more questions and consequences than I first thought !
