I agree that regular Militia regiments were not offically transferred to the new Volunteer Regiments , but there does appear to be some evidence that in several cases, especally in the spring sumer of 62 that we find most of the members of a former militia company enlisting together in a newly forming unit. I have several examples from the Gerdes Civil War Page:

Soldiers from the 1862 Muster roll of the 7th Militia Regiment enlist in 34th Arkansas Infantry Regiment, CSA also several enlisted in Union units
Soldiers from the 1862 Muster roll of the 10th Militia Regiment enlist in 25th Arkansas Infantry Regiment, CSA and the 4th Arkansas Cavalry also several enlisted in Union units
Soldiers from the 1862 Muster roll of the 21st Militia Regiment enlist in 36th Arkansas Infantry Regiment, CSA also several enlisted in Union units

There are several other examples. I have always assumed that this had something to do with the new Consription Laws. I imagine that the former Militiamen decided that rather than waiting to be conscipted, it would be better to enlist with their friends in a new unit.

I think Anthoy Rushing has some evidence that some Saline County militia units enlisted pretty much intact, even keeping the same officers, but I am not sure which units he is tracking.

I wondered if your data base might show show soldiers Militia Unit and thier Volunteer unit some some comparisons could be made?