Not going to happen anytime soon because 5.56 is a good cartridge and they still have to fix the weakest link, the shooter. 5.56 is a great all around cartridge especially with the new M855A1 rounds. It's cheaper, lightweight, less recoil, plentiful worldwide (NATO), and a good compromise for close quarters and long range engagements. Going with something more powerful like a 123gr results in more recoil, less control in full auto, and reduces your ammo loadout by almost half.
Better thing to do is give better marksmanship training to the troops for longer ranges and shot placement for CQB. Things like CCOs and ACOGS have improved marksmanship and Ithey'll issue out some sort of electronic gizmo to make accurate shots even easier in the future. Maximize what we got now before we switch to something else. I'm talking about the basic rifleman issue stuff. There's some instances where a new round would be good to have for special circumstances. Squad designated marksman, special ops, etc.
Time a caliber change when they come out with a revolutionary change in guns. Electronic ignition, caseless ammo, etc.