I ask this question because there are a very few things that actually decrease Body in Unknown Armies (damage of 50 wound points or greater being one of them, Herpemancer spells being the other) and several Epideromancer spells that permanently increase either Body or Wound Points, but not both at the same time. My question is this: If something drops my Body value, either permanently or temporarily, does my Maximum Wound Point value drop along with it?
Good question. As far as I can tell in UA2nd Edition there is no explicit information regarding the effect of a drop in the Body stat.
Page 36 (on stats) discusses the effect of each level of body, and the derivation of initial Wound points from the Body stat and how to handle increases in both through XP spent on Body, but does not mention a subsequent decrease. It does, however, explicitly state that increases in Body via magic do not likewise cause an increase in Wound points.
From this explicit statement that magical increases in the Body stat are unrelated to the pool of wounds, we can infer that there is a similar disconnect when magic decreases the Body stat and focus solely on the roleplaying and descriptive effects of the new level of the stat. The information on permanent damage after major injury on page 59 reinforces this conclusion by listing a range of things which might occur as a legacy of such wounds. Among the items on the list are permanent decreases in specific stats wherein a loss of Wound Points and a reduction of the Body trait are given separately.
It would certainly be within the GMs purview to have a reduction in the Body trait cause a reduction in Wound Points, but it does not seem to be a requirement or a rule of the game that this occurs.