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