R900 memory upgrade - memory paring placement

I have an R900 with 128GB of RAM (4GB DDR2 667 FBDIMM  x 32 sticks, split between four risers).  I'm retiring some 2950's which have 8GB sticks of the same type (DDR2 667 FBDIMM).  According to the hardware guide, the R900 memory is arranged in 16 banks of pairs.  What i want to know is as i start replacing the 4GB sticks with 8GB sticks, what is the mapping strategy?

let's say i just want to swap in one pair of RAM. 

is the pair Riser A, position 1 and 2, or is the pair Riser A, position 1 and Riser B position 1?

then for the next pair, can i keep installing in Riser A and Riser B or does the next pair have to go into Riser C and Riser D

Thanks in advance

beekerc