“In-Flight Replanning” referred to a scenario where you couldn’t legally dispatch as you would arrive at destination with less than the required amount of fuel, because for whatever reason(s) you couldn’t get any more on. To get round this, you could file to somewhere closer that was legal, in the expectation that direct routings, optimum levels and the relaxing of in-flight fuel criteria as you got closer in time and distance to where you actually wanted to go would allow you to carry on, but if that wasn’t possible you would tech stop somewhere convenient near the end of the flight.
This is different from the original scenario because your filed and intended destination are the same in that case, whereas with this method you are changing your destination in-flight if/when it becomes possible.
We don’t do this any more so I don’t have any up-to-date info but I presume some carriers might still utilise this in extremis.