Moving the evening, once, for nothing
A booking can be shifted to another date once at no cost, provided the request arrives with more than a day to spare and the new date is within a fortnight.
Short trips rearrange themselves. One move of date is allowed for nothing, and on a two night break it is usually all anybody needs, because the only realistic alternative to the Friday is the Saturday.
The two conditions
The request has to reach the thread more than twenty-four hours before the hour that was booked, and the new date has to fall inside the next fortnight. Satisfy both and the front half travels across whole; nothing is deducted and nothing further is asked for. A two hour Friday held with its front half becomes a two hour Saturday held by that same front half, not a cent more or less.
What a second move costs
A second move, or a first one sent inside the final day, stops being free and is measured on the notice scale exactly as a cancellation written at that moment would be. Whatever the scale does not keep goes forward onto the new date rather than coming back to you, so the booking survives; it is simply smaller.
Using it deliberately
Because the move is free, there is an argument for booking the earlier of two candidate evenings rather than agonising over which. Hold the Friday, and if the weekend turns out to want the Saturday instead, move it on the Thursday for nothing. The booking terms set the same rule out in full, and a delayed flight can sometimes be handled this way rather than as a cancellation.