using 2000 it isn't that easy to do this in situ so you need to do the changes within the block's dwg file. Open the block rather than inserting it. This will give you access to the items you need to change without exploding anything. When finished save the drawing and it can be inserted into new drawings as you wish.
If you need to update existing drawings you need to redefine that block. Start the INSERT command but instead of selecting the block from the drop down list browse for the newly redefined block. Whan asked if you want to redefine say yes. Once you see the redefine happen you can <esc> the insrt command.
There are easier ways with newer releases.