I don't understand why you are assuming they were Virginia State Line prior to the 7th Bn. BTW, I have no indication that there ever was a "7th Bn Va Cav" and as I stated, Prentice was LtCol of the 7th Confederate Cavalry which is a different unit altogether.
The 21st Va Cav was composed of former Virginia State Line members. It was organized 08/27/1863 and disbanded 03/31/1863, having been raised in localities where the Conscription Act was not enforceable.
"21st Cavalry Regiment was organized in August, 1862, with companies which had served in the Virginia State Line. The unit was assigned to W.E. Jones' and McCausland's Brigade, and in April, 1864, it contained 317 effectives. It took an active part in various conflicts in East Tennessee, western Virginia, and in the Shenandoah Valley. During mid-April, 1865, the regiment disbanded. Its field officers were Colonel W.E. Peters, Lieutenant Colonel David Edmundson, and Major Stephen P. Halsey."
There were 8 Sutherland's in this unit:
Soldier Name Company Rank In Rank Out
Sutherland, D.T. Co C Private Private
Sutherland, Ezekiel Co E Private Private
Sutherland, Harrison Co E Private Private
Sutherland, Johnson Co E Private Private
Sutherland, Noah Co E Private Private
Sutherland, Samuel Co E Private Private
Sutherland, Thomas Co C 2 Lieutenant 1 Lieutenant
Sutherland, Van Co E Private Private
Co C(2nd) = Capt Alex C. Branscom's Co; formerly Co G(1st); organized 07/17/1863
Co E(1st) = Capt E.K.Count's Co; also reported as Co K; organized about 04/01/1863 and stated by Col Peters in a letter dated 08/26/1863, to have been composed of men formerly i State service (Virginia State Line); Capt County was formerly Capt of Co B, 2nd Regt Va State Line; the Capt & 1st Lt were dropped from the rolls dated 04/26/1864; the Company probably disbanded as there are no further records
Co E(2nd) = Capt C.F. McDonald's Co; formerly called Co I(1st) and Co H(?); organized 08/01/1863
Coming from State Line Service is not the usual route. The usual was coming from a militia unit.