Because on most Cisco hardware when you update an access list your new commands are appended to the end of the list. This is different on ASA firewalls where entries are line numbered so you can insert at a specific point in the list, a bit like on Checkpoint.
What you will need to do i am afraid is take a copy of the current list into Notepad or Wordpad, edit the list to insert your new rule in the correct location, remembering that an access list is always executed top to bottom and will match on the first hit, irrespective of what rules follow.
Next you will need to delete the access list from the running configuration and re-apply the new list. It's a while since I have had to do this on a 2600, but I am fairly certain you will need to re-apply the list to whatever interafces it was on.
CAUTION:
1. Make sure you get the order of the access list correct so you do not cut yourself off from the device in question
2. When you delete the old access list the rules are not held in memory so traffic will flow freely until you apply the new list.
and finally - sorry for stating the obvious but take a configuration backup before you start
wri mem AND
copy start tftp etc
All the best
