It depends on how different the notifications are. If the changes are not too significant, you can set a boolean flag (default being true) in the process once the first round of approval has been done (set to false) and then use that flag in an If Inline function. So the True part will return the value for the initial mail and the False, the subsequent values.

If we are talking about a major structural change, it might make sense to use a Mail event instead (new event prior to the InfoPath Client event) and use code behind to change the body of the mail being sent out. If you are unsure on how to build the URL needed, just have a look at the code behind for the InfoPath Client event, use the SetWorklistUrl method.