Sounds like there is a very interesting story in here somewhere. W. J. Deramus enlisted in Co. K, 19th La. Inf. on December 15, 1861, and on May 12, 1862, was elected 4th Sergeant. He seems to have been present on all the rolls, but on January 30, 1863, he was busted back to private. And now you tell us that in the spring of 1864 he was serving with the Yankees in the 1st La. Scouts. He wasn't unique in serving with both a CS unit and the 1st La. Scouts, but it does seem unusual that he made it to the NCO ranks and then went over. Something serious must have happened to disillusion him.
Concerning whether he is buried under a US or CS marker: Someone definitely needs to check on that. If he's buried with a CS gravestone, it needs to be removed. Whatever old Wesley's reasons might have been, I doubt that deserting to the enemy qualifies as honorable service in anybody's book.

Deramus, W. J., Pvt. Co. K. 19th La. Infty. En. Dec. 15th, 1861, New Orleans, La. Present on all Rolls to March, 1862. Roll for April and May, 1862, Present. Elected 4th Sergt., May 12th, 1862. Rolls from June, 1862, to Dec., 1862, Present. Roll for Jan. and Feb., 1863. Present. Reduced to ranks, Jan. 30th, 1863.