Dubya is probably murmuring in Tony Blair's ear, "Shut up, Tony!" He needs a frank discussion about nuclear force modernization like he needs a full-blown migraine.
US DoD has done that conversion/upgrade to their "dial-a-nuke" warheads, making them over into ground penetrators with variable yields, but that's about it -- other than the constant build-down and Trident => SSGN conversions.