Grendel carbines and DMR rifles actually weigh a few ounces less than 5.56mm guns because the bore is bigger and there is less barrel steel to tote up the hill.
What would it take to convert millions and millions of M4 and M16 rifles to 6.5 Grendel? In theory, just a barrel, a bolt, and a magazine. The cartridge produces more recoil than the 5.56, but not so much more that it requires a new buffer or scope/accessory mounts.
The M4 5.56 platform for the military isn’t going anywhere. They MAY move to a different round, a slightly different rifle style, and probably new suppliers for a standard issue battlefield rifle, but the 5.56 AR will survive if only in the shorter barrel rifle outfits. There isn’t much better than a 10.5” H&K 416 for short range (close quarters) clearing. It is also the perfect home defense weapon IF you are trained with the rifle. If Trump and co. would simply repeal the NFA, we could all have one without govt. interference.