Originally Posted by
corporate-wage-slave
If you no show then your ticket becomes invalid, so you need to be able to call BA, change online or do it at the airport at least 1 hour before the departure time currently associated with your ticket. You can make several changes but best to avoid that it possible, it can get messy. Your best bet is to stick to a flight you know you can make, but if you get to the airport with more than an hour to spare for an earlier service then by calling or queuing up you can get yourself brought forward. More information here:
https://www.flyertalk.com/forum/brit...change-ba.html
Agreed ^ Also to add to
cws's answer about the case of multiple changes, whilst there is no limit to how many times you can change the flight on the day (and I have indeed occasionally done it 3-4 times though the 1 hour rule is crucial here!) in my experience, after the first change, the online change facility often shows errors or tries to charge you which means that you need to call (which can be awkward if you are flying from a non-EU airport and roaming charges apply).