If it is a title block in say layouts then can change all relevant title block attributes, but this is done outside of Sheet sets. Simplest way is open dwg and can change the values.
; modify all blocks 1 attribute
; By AlanH March 2021
(defun c:attmod ( / x bname ans att ent ent2)
(vl-load-com)
(setq ans (getstring "\nEnter new string "))
(setq ent (nentsel "\nPick block attribute Enter to exit"))
(setq tagname (cdr (assoc 2 (entget (car ent)))))
(setq pt (car (cdr ent)))
(setq pt (list (car pt)(cadr pt)))
(setq ent2 (ssname (ssget pt) 0))
(setq bname (cdr (assoc 2 (entget ent2))))
(setq ss (ssget "X" (list (cons 0 "INSERT")(cons 2 bname))))
(repeat (setq x (sslength ss))
(foreach att (vlax-invoke (vlax-ename->vla-object (ssname SS (setq x (- x 1)) )) 'getattributes)
(if (= tagname (strcase (vla-get-tagstring att)))
(vla-put-textstring att ans)
)
)
)
(princ)
)
(c:attmod)
And this one for layouts fill in the new values.
; Change all title blocks based on new attribute values
; By Alan H June 2024
(defun C:ATTMOD2 ( / obj bname att atts lst ans ss blk x)
(setq obj (vlax-ename->vla-object (car (entsel "\nSelect title Block (attribute) "))))
(setq bname (vlax-get obj 'Name))
(setq atts (vlax-invoke obj 'Getattributes))
(setq lst '())
(foreach att atts
(setq lst (cons (vlax-get att 'Tagstring)lst))
(setq lst (cons 20 lst))
(setq lst (cons 19 lst))
(setq lst (cons "" lst))
)
(setq lst (reverse lst))
(setq lst (cons "Please change" lst))
(if (not AH:getvalsm)(load "Multi Getvals.lsp"))
(setq ans (AH:getvalsm lst))
(setq lays (layoutlist))
(foreach lay lays
(setvar 'ctab lay)
(command "Pspace")
(setq ss (ssget "X" (list (cons 0 "INSERT")(cons 2 bname)(cons 410 (getvar 'ctab)))))
(setq blk (vlax-ename->vla-object (ssname ss 0)))
(setq atts (vlax-invoke blk 'Getattributes))
(setq x 0)
(foreach val ans
(if (= val "")
(princ)
(vlax-put (nth x atts) 'Textstring val)
)
(setq x (1+ x))
)
)
(princ)
)
(c:ATTMOD2)