Just to let you know - there is an established British Airways thread in this forum currently on the next page so no need to start another.
The answer to your question is: How long is a piece of string? The price will depend upon (amongst other reasons):-
- The booking class of the ticket defines whether it can be changed at all.
- The booking class defines how much a change will cost.
- The difference between what you booked and what you want.
- How many seats are available on the flight/s that you now want.
The only way to find out is to visit the place where your ticket was issued. If you bought on-line direct from BA, then you will have to phone the BA customer service desk to find out.