This happens fairly often since airport taxes and other charges change day to day (with a difference this small I don’t suppose the fare basis changed).

In the first case (tax change) you can ask one of the segment to be rebooked (e.g. change of time or day). This will trigger the system to recalculate the fare with the same fare basis used at time of purchase, but taxes should be updated and on full flex fares they get refunded automatically.

In the second case (fare basis change) you need to ask for a rebooking of the first segment only. This will trigger the system to recalculate the fare with the fare basis available at the time of rebooking.

This all supposes that you rebook into the same booking class of course, otherwise fare difference may apply.

I book complex multi-city in full flex all the time, the above has worked well for me. Do note that it can get messy if your itinerary is really complex and if you don’t have SEN/HON competent people to deal with it… especially when the fare basis doesn’t exist anymore and they need to know how to properly reprice the fare based on historical data (supposing the first segment doesn’t change).