A few points to amplify an excellent post. With the passage of the Conscript Act in early 1862 -- the first draft law on the American continent, inbcidentally, Confederate units were reorganized, electing new company grade officers. Most of the men had enlisted for one year, so many of their terms of service were expiring at that time. For those between the ages of 18 and 35, they were required to either reenlist in their original units, or else immdeiately join other units. If they failed to do so, they were subject to conscription. Many of the men took the opportunity to go home for a brief time, and failign to reenlist, became subject to conscription.. So Bryan is absolutely correct that many of those conscripted in 11862 had prior service.

However, througout the war the age for those subject to conscription was several times readjusted to expand the manpower pool. By late 1864 it was expanded to 45 as the upper age for liability for conscription in regular units, and up to 55, if I recall correctly, for reserve servcie. The lower age remained at 18 for regular service, but was reduced to 16 for the reserves.

Many men had been discharged form the service for disability form wounds or disease. However, as the war progressed, the Medical Examining Boards and surgeons working for them took an increasingly stricter view of what constituteddisability, and so many men who had once been determined "disabled" were reevaluated -- it was a normal process, much like social security disability today is supposed to work (and usually doesn't)-- and determined to be available for conscription.

My experience is that a substantial number of those conscripted in 1864 and even early 1865 had prior service, had been discharged for a number of reasons, and then got caught in the net of the conscription process as the demand for manpower became critical. I think the safest course is to assume prior service; obviously many will not have it, but I think you'll find that the majority, other than the 18 year olds entering the pool for the first time, did have prior service. And late in the war many of the 18 year olds will have service in a reserve, state troops, militia or similar units. Often the real problem is identifying those units, because the records for many reserve and militia units are particulalry sparse.