Nike Ajax - given that it was replaced with the Nike Hercules, there is no real point in thinking about upgrades for it. About the only possible item was to reduce the size of radars/support systems and sell to foreign countries as a point defence system. Liquid sustainer would be its main problem and would need to be replaced with a solid one. However, other missile systems came along to replace it (SAM-D/Patriot eventually!!)
Nike Hercules - improvement in radars and electronic processing. Multiple target engagement. Again, it was fixed system, but it had some mobility. Smaller radars would have helped, but it was still a big missile to move, thus SAM-D was a better option, and eventually comparable range and one would assume same/better kill probability.
Nike Zeus - Two options here. The A version (a bigger Hercules essentially), and the B (which became the Spartan eventually). These were specifically ABM missiles, not anti-aircraft so again, not much point in thinking of upgrades. However, as this is alternative history, lets speculate a little. The bomber threat at this point was almost over (mid-60s, but then again, Linebacker). So how about a MIRV'd type of SAM? It would still be good against waves of incoming bombers. (Guidance would be interesting, but some good IR would help and seekers were becoming decentish in this timeframe.) Launch main missile up high, loiter?, then discharge sub-missiles/munitions to fall down and home in on individual targets. Could do the same for cruise missiles as well (look down IR could be possible??). As I said, just dreamin...
Regardless the Zeus missiles would be fixed based, but had the potential to cover a lot of ground from one site.
As for an ABM version, that went through SDI (in a manner of speaking), so they needed to get rid of the nuke warheads, make them a hit to kill, and ensure appropriate guidance for launch, acquisition and terminal control. Again a multiple warhead for the Zeus missile, and with improved booster/sustainer, that would overcome the extra weight that would be involved. This is assuming that you have a wave of incoming RVs, rather than a single rougue type scenario.
