I think I'd probably have left them as is, and put the new sticks into the remaining sockets. Paired memory can also be done over interleaved sockets and I suspect that's what has happened.
Having said that, performance is not likely to be affected in any noticeable way and if you can see all 24gb of ram, I wouldnt worry about it.
That's what I did with my new iMac....
24 GB seems to show up just fine and I haven't had any problems.
Jaime T. Matthew