OK, I think I have a possibility here, LT is not going to insert the new block definition unless you use DC or you delete and purge the original.
The nice thing is we can probably get the somewhat automated depending on a few factors.
First off, in the viewport where your block that you need to update, is there ever anything besides the block located there?
This is important to make this work the way I am thinking it could.
Let me know, I don't wanna start playing with the code till I know.
-Chris