But If You Swap & Change Whos Who ????
Forums that allow name changes usually have ways to ensure continuity. For example, they might show JoeBloggs (formerly Bill Smith) for a month after the change, and provide a history link for at least a few months thereafter. So, there is a planned transition from one name to the other, not an abrupt switchover.
Currently, giffgaff sidesteps that requirement by only implementing name changes for new members, who would mostly not yet have built up much of a public presence and therefore it hasn't so far been necessary to implement that.