This is a classic issue with AutoCAD. Though they are part of the block definition, attributes are actually their own entity in the database. So when you redefine a block to have a different tag name, all new instances of that block have the newly named attribute while all previous instances have the old name.
What you need is a custom program the walks through the drawing collecting the values of the "old" attribute, performs an ATTSYNC, and then goes back and puts the values back into what basically amounts to are new insertions of the block. Every AutoCAD programmer has had to write their own version of this program.
