The M249 did replace two M16s in each squad. We hadn't really had a squad automatic rifle since the BAR. The M14E1 doesn't really count because it wasn't fielded in any quantity.
We are already replacing the M60 with the M240B. Fielding started about a year ago. Ranger Regiment, 82d Airborne and 101st Airborne (Air Assault) already have them. Other units may have them by now. The M240 has been in the American system since the late '70s. It was first adopted as the coax gun on the M1 Abrams tank. It was also the coax on the M2/M3 Bradley Infantry/Calvary fighting vehicles. There was a ground mount kit purchased that allowed the crews of these vehicles to dismount the machine gun and use it in the handheld mode. The kit included a skeleton type buttstock, a pistol grip and trigger group and bipod. I'm not sure if these kits were ever fielded or kept in war reserve.