That is an odd change, I know the RPD is the older design, 1945 or so, and wikipedia claims the RPK was adapted to attempt standardize their logistics, but a belt fed MG is generally preferable to a box magazine gun, at least in my opinion.
However I can see some benefits in favour of the RPK, it's 6 lbs lighter, and uses nearly all the same parts as a Rifleman's AK, but the long magazine options do keep teh gunner from getting very low to the ground.