I don't yet have experience with WS33700137. I suppose this is the "new" leave request workflow?
I am a bit confused that what you mean by notifications? Do you mean the approval work items? What do you want to happen when the employee changes the leave?
Here are couple of pointers that might lead you to right directions:
First of all, at least in the old leave request workflow things worked like this: If employee sent a leave for approval, but changed it before the manager approved it, nothing really happened (because the leave wasn't yet posted to the infotype). No new workflow was started, and the same approval work item stayed in the manager's inbox. BUT if the manager had approved the leave, and it was posted to the infotype, then if the employee changed the leave, a new workflow was started. (You might want to check if this is something that you can change in the IMG).
Secondly, you should probably check the BADIs of the leave request functionality. There are some methods that you can put your own code into, so perhaps you can do whatever you are looking for in one of these methods (=>When leave request is being changed, etc. But please pay attention to what I said earlier: Changing a posted leave request is a different thing compared to the pending leave.).
Kind regards,
Karri