I bought my AR rifle during the mid 90's, as a parts gun. The lower is an Eagle Arms EA-15 (Coal Valley), and it is mated to an unmarked A1 receiver and 20" barrel. The rifle functions just fine,  but I thought I would "upgrade" it by replacing the upper with a newer type. I tried test-fitting a Rock River upper this evening, but the two pin holes did not match up, and there is a noticeable gap between the two sections. Does anyone know what brand upper should fit with this lower?
thanks in advance.

You will have to actually try them.. Most SHOULD fit, but if they dont you can always work the upper a bit to make it fit..

Dont modify the lower.. Work on the upper.

I could be wrong, but for a while RRA uppers were having trouble fitting other brands of lowers.  I've read about this a million times, and we had to do a little work to get my dads upper to fit his E.A. (Essential Arms, not Eagle) lower back in the ban days.  I think most other brands should work. YMMV.

I have a Coal Valley Eagle arms lower, excellent quality.

All in spec uppers should work.

Thus far I have tested the following on mine

Bushmaster A3
Colt A2
CMT A3