Thomson Holidays -Flight Conditions of Carriage
ARTICLE 9 - SCHEDULES, DELAYS, CANCELLATION OF FLIGHTS
9.1 SCHEDULES
9.1.1 The flight times shown in timetables may change between the date of publication and the date you actually travel. We do not guarantee them to you and they do not form part of your contract with us.
9.1.2 Before we accept your booking, we will notify you of the scheduled flight time in effect as of that time, and it will be shown on your Ticket. It is possible we may need to change the scheduled flight time subsequent to issuance of your Ticket. If you provide us with contact information, we will endeavour to notify you of any such changes. If, after you purchase your Ticket, we make a significant change to the scheduled flight time, which is not acceptable to you, and we are unable to book you on an alternate flight which is acceptable to you, you will be entitled to a refund in accordance with Article 10.2.
If they will give you a refund then can't see them refusing to change you hoildays once its not above the cost of the current one.