It is my understanding that the Coast Guard already has a few HU25's in storage at Davis Motham (Boneyard). While it may be a few years before the last one is parked, it is coming. As for a 731 conversion, it isn't economicly fesable. Besides I don't think the 731 has enough growth in it to push that heavy an aircraft. And the 200 is heavy.
Avcon has a mod for the Lear 35 and 36 to expand the fuel load about 800 pounds. They expand the tip tanks about 18 inches. The full mod also adds delta fins and increases the max take off and landing weights. It may also increase the zero fuel weight. Garrett has a mod to the 731-2 to increase the power, I believe it is the -2C mod. And I have heard that there is an STC in the works to put -3's on the 35 and 36.