How do I replace one block with another throughout an AutoCAD drawing?

What cadssistant does

1. Checks the block table: both names must exist. If the new one is missing it asks where to insert it from, or you insert one copy first.
2. Selects every INSERT named CHAIR-OLD and rewrites the block name to CHAIR-NEW in one AutoLISP pass, which keeps every other property of the reference.
3. Replies with the count. Say "purge CHAIR-OLD" afterwards to drop the old definition once nothing references it.

The same task in AutoLISP

(progn
  (setq ss (ssget "_X" '((0 . "INSERT") (2 . "CHAIR-OLD"))) i 0)
  (if ss
    (repeat (sslength ss)
      (setq ed (entget (ssname ss i)) i (1+ i))
      (entmod (subst (cons 2 "CHAIR-NEW") (assoc 2 ed) ed))))
  (if ss (sslength ss) 0))

CHAIR-NEW must already be defined in the drawing. Attributes stay as they were, so a new block with different attribute tags needs ATTSYNC afterwards.

Good to know

• The base points of the two blocks must agree, or every copy shifts by the difference. Say "check the base points" and it compares the two definitions before swapping.

More in edit in bulk

• How do I move all objects of one type to another layer in AutoCAD?
• How do I change the height of all text in an AutoCAD drawing at once?
• How do I delete everything on a layer and purge it in AutoCAD?
• How do I rename many layers at once in AutoCAD?