It's upto Thai, they may take pity on you and allow a free change, I don't know what kind of ticket you have, but if it is a non-changeable be prepared to pay.
You won't be able to get anything out of BA if you have seperate tickets as they will have fulfilled their contract with you, remember that time forms no part of your contract. It's in section 9 IIRC of the conditions of carriage.