Difficult because of course Japanese pacifism wasn't originally voluntary - it was the policy of an occupying power under the terms of unconditional surrender. As a result, there has always (and always will be, in the foreseeable future) a revanchist Japanese right wing that vigorously opposes pacifism, and there has consequently always been a liberal wing that upholds pacifism. The presence of an army, not that army's neutrality, is the big sticking point. So you'd need to change the fundamental premise of the issue to change attitudes in the way you want - which, of course, is implied by any "reverse" AHC.
You'd need to find a very specific point in history: a time between the US keeping close official watch and the total normalisation of the "New Japan" that ensured the continued dominance of the liberal position, at least up until the crash. I'd say that's sometime in the 1960s. Then you would have relatively inoffensive nationalist elements infiltrate the LDP; the roundabout way, but maybe the most plausible, might be to bolster the Japanese left, and spook the Americans enough with that to make them
encourage the rightward drift of the LDP.
So I think the perfect time for this would have been the 1968 protests, part of the global shitstorm that put the CIA on edge and empowered right-wing reaction everywhere on Earth. Have Japan's 1968
Anti-War Day become an even bigger deal (apparently 4.5 million people showed up to the protests, but like all the '68 protests it eventually descended into disorganised violence), and then get the CIA, thinking that the liberal faction hasn't done enough to quell Socialist sentiment, to swing from Japanese pacifism to moderate Japanese nationalism. LDP drifts rightward -> rightward politicians advocate for a standing army, amend the constitution -> armed neutrality as a
possible, but maybe still unlikely, consequence.