Suppose that on January 1, 1864, Ulysses Grant falls off his horse, hits his head, and dies. Lincoln decides to appoint George Thomas, commander of the Army of the Cumberland, to the command of the Army of the Potomac.
Assuming that the forces are more-or-less the same as they were in the 1864 campaign of OTL, how would a matchup between the Army of Northern Virginia under Lee and the Army of the Potomac under Thomas turn out?