ATTSYNC

ATTSYNC is a native AutoCAD command that updates existing block references to match their block definition's current attribute definitions (after attributes were added or removed in the block editor). It only syncs the attribute structure — which fields exist — not each reference's values, and it's a known trap: run it carelessly and it can reset or wipe out attribute values that were already filled in, because it isn't designed to preserve arbitrary existing text against a changed template.

Tools built specifically for pushing a redefined block out to many drawings account for this failure mode explicitly, rather than relying on ATTSYNC alone.