Well, you can stop at step 3 (where you give them a new LimeLM key, but you haven't told them about it). That is, your scripts using our web API will handle the old key to new key transition.
We simply recommend notifying customers of their new key so that if in the future you don't want to maintain that "shim script", you can drop it (and just tell them to use the new key).
But, long story short, it's up to you how you want to roll it out. If you don't want to email your existing customers, you don't have to.
Let me know if you have any other questions.