I'm responsible for keeping our symbol library updated.
My method for this situation has been to use REFEDIT, make necessary changes, including using the Symbol Builder, but NOT using the Block or Wblock functions in it; use it only for quicker access to the building blocks.
Then I use REFCLOSE, save changes, use WBLOCK to write the new definition out to disk, then use the Swap Block/ Update Block/ Library Swap tool to select and replace the block with a new copy (this step is to get new attributes placed properly.) I find this to be the quickest way to permananently affect block definitions. The Add Attribute function is nice for one-offs, but I generally need something that's more permanently re-usable.