I personally hate it when someone has the same issue as me then posts "I fixed it!" and doesn't post their code...
Here is the final code:
(defun c:test ()
(if (setq T3 (ssget "_X" '((0 . "DIMENSION")))) ; finds ALL dimensions
(repeat (setq n (sslength T3))
(setq tdata (entget (ssname T3 (setq n (1- n)))))
(entmod (subst '(3 . "NewDimStyle") (assoc 3 tdata) tdata)) ;;;Assumes NewDimStyle exists / replaces the old Dimstyle with the new
)
)
(princ))
Thank you again Steven.