The 8th NY Cav, one of the best regiments in the Union Army, didn't receive its horses until the summer of 1862, but they managed to escape from Harper's Ferry in Sept 1862 and do just fine at Gettysburg. In Custer's Michigan Brigade, the 5th, 6th, and 7th Mich Cav Regts had less than a years service before they, with the 1st Mich Cav, fought at Gettysburg. What took 2 years was for good cavalry leaders to come to the surface.

All regular Army regiments were undermanned. Details from volunteer regiments were used to fill artillery batteries, but I have never read that they were used to augment cavalry regiments. What happened in the Army of the Potomac was that the Reserve Brigade of the Cavalry Corps, to which the 4 Regular Army cavalry regiments in the East were usually assigned was so reduced that whole volunteer regiments were assigned to the brigade. The 6th Pa was with the Reserve Brigade from beginning to end, the 19 NY for a few months and the 2nd Mass for the last year or so. At the end even with the 6th Pa and 2nd Mass the brigade mounted less than a thousand men.