Tim,
You should be able to accomplish what you want to do fairly easily with a lisp routine however, there a couple of "gotchas" you may want to consider. First, you mentioned you want to replace the leader by selecting 3 points and then have it erase the old leader, and create a new one. You may consider selecting your 3 points in addition to the text so you don't have to re-type anything. Second, you may want to consider selecting the leader line itself, and a start point instead of the 3 points to create a leader - this way when you delete your old leader you will be assured you are deleting only the leader you want and not any other geometry, and you'll have the luxury of actual entity information. Finally, the reason you'll want the entity information from above, you mentioned you have a large amount of drawings to do (and for me anyway), if I get interrupted for any length of time, I have a tendency to forget where I left off. If you are selecting the entity and therefore have its information, your program could warn you, or give you the option of replacing the new style of leader so you're not wasting your valuable time.
HTH
Todd
