Japan(or, for the matter, South Korea), as of now, are more of very well developed footprints for US presence, with appropriate local defense capabilities offloaded from US budget. Both form rather inconvenient(from China's perspective) geographic "barbican", effectively limiting China's north-eastern access.
Both have to swing very hard, breaking all the American entrancement overnight, just to get out, which is highly unlikely (not impossible, but usually such a system makes such ideas highly unpopular/personally harmful in the first place). Both rapidly develop conventional/possess sub-threshold nuclear deterrent capability.
Both have potential to militarize(even if once, because with current demographic picture no country can truly afford go 20th century). RK can do it faster due to better positioned industry (but within a limited window of time (demographic clock), Japan will take longer, but their downward spiral is also more sensible.
Japan clearly seemed to overtly prepare for possibility of such a shift for last decades, but within their current alignment it does not work.
Can both be suppressed fast? ROK, potentially(geography; DPRK agreement/commitment highly desirable), Japan, less unlikely(geography; at least DPRK or better Russian commitment is highly desirable, but Russia is likely to be reluctant here, it doesn't have a strong hand in Far East against Japan).
Can both truly be expected/threatened into remain neutral?
That's almost an impossible guess beforehand, i.e. 50/50.