The express tool fr global attribute editting is slick, but you can do it
with plain AutoCAD as well.
Command: -attedit
Edit attributes one at a time? [Yes/No] <Y>: N
Performing global editing of attribute values.
Edit only attributes visible on screen? [Yes/No] <Y>: N
Drawing must be regenerated afterwards.
Enter block name specification <*>: "press enter or keyin block name"
Enter attribute tag specification <*>:"press enter or keyin tag name"
Enter attribute value specification <*>:"press enter or keyin existing
value"
Enter string to change: "enter what you're looking for"
Enter new string: "enter what it should be"