Hi Kobi,
When the process finds the next approver(s), our email notification policies can catch this timing and send an email.
However since this is not a new activity in the process (i.e. the approver was previously assigned but is now changing outside of the process, meaning no new step is introduced), the only solution to this question will involve customization, and a Support case will need to be opened.
There might be a way to write a BLTH on the approval task - In the scope of the Support case, we will follow the modifications of the approval task in the TP and WPDS databases, and see if there is a timing we can catch and hinge the code on.
Hope this helps,
Einav