Found this one years ago and I have no idea who the credit goes to.
; Match scale of one block to another
(defun c:mbl ()
(setq fb (entget (car (entsel "\nSelect Block reference: ")))
fb41 (assoc 41 fb)
fb42 (assoc 42 fb)
)
(setq sb (entget (car (entsel "\nSelect target Block: ")))
sb41 (assoc 41 sb)
sb42 (assoc 42 sb)
)
(setq sb_ent (subst fb41 sb41 sb))
(entmod sb_ent)
(setq sb sb_ent)
(setq sb_ent (subst fb42 sb42 sb))
(entmod sb_ent)
(command "regen")
)