If you are trying change the Z axis only :
(defun c:3df-zaxs (/ z ss i en ed)
(initget 1)
(setq z (getdist "\nNew Z Value: "))
(while (setq ss (ssget (list (cons 0 "3DFACE"))))
(setq i 0)
(while (setq en (ssname ss i))
(setq ed (entget en))
(foreach p '(10 11 12 13)
(setq ed (subst (list p (cadr (assoc p ed))
(caddr (assoc p ed))
z)
(assoc p ed) ed)))
(entmod ed)
(setq i (1+ i))))
(prin1))
Note that all 3dface values are WCS
-David