reconfigure the M4 to fire a 6.5mm grendal.
That's probably what will happen. We'll either get a new platform (more costly) or a modified M4 with a new caliber.
I say they need to look at the new 7.62 x 28 WSSSM (Win Super Super Short Mag) /s
And while you're replacing M4 Uppers, replace the direct gas impingment system with a gas piston system ala HK416 and others.
The M14 is not practical for today's mounted urban environment, which is why the M4 has displaced the M16. Can you imagine egressing an MRAP with a 46.5 inch long M14?
The M14 is appropriate for Designated Marksman and SOCOM use as mission dictates, but not as a universal infantry weapon.