AT my location, Dreamland, the reason for the significant change flashes for a period of time on the controllers display screen. So if you call up with the previous letter, the controller can tell you the information has changed and what the change is.
The problem with the equipment is that the change does not flash for long enough sometimes to be noticed (this is, we hope, going to be changed) and if you are more than one letter out then any previous significant changes may have been lost. I am currently suggesting that a line on the screen that is very infrequently used be utilised to type in what the last significant change was so that it is permanently available to the controller (or at least until the next change!)