(defun c:ln (/ ds dc sl m ls dy) (vl-load-com)
  (if (and (setq ds (getdist "\nOld distance:")) (ssget '((0 . "line")))
        (setq dc (vlax-get (vlax-get-acad-object) 'activedocument)
          sl (vla-get-activeselectionset dc)))
    (if (and (vlax-for m sl (if (= (rtos (vla-get-Length m) 2) (rtos ds 2))
          (setq ls (cons m ls)))) (setq dy (getdist "\nNew distance:")))
      (progn (vla-startundomark dc)
        (mapcar '(lambda(a) (vlax-put a 'EndPoint (polar (vlax-get a 'StartPoint)
          (vla-get-Angle a) dy))) ls) (vla-endundomark dc)
      ) (alert (strcat "No line found " (rtos ds 2) " units long"))
    )
  ) (prin1)
)