Hi,
I am no expert, we only try to help each other out from our different experiences,
1. If getting a new Employment reference won't be stressful just get a new one for EE ITA when it comes
2. If it will be cumbersome, you can simply attach the one you already have as it is, adding an LOE means that the reference will no longer be valid
so don't see how that helps.
My honest opinion is that you get a new one from the new person (Supervisor) when the time comes, if it's someone you don't know, politely ask the supervisor you know (The one leaving) to introduce you to the new person ahead.