I found this on Virgin Holidays website. Might be worth giving them a call or drop them an email and talk this over with them:
Q: I want to change my travel date
A: If you're looking to change your departure date or destinations, as per our terms and conditions, this will be taken as a cancellation of your original holiday contract with us and we'll need to make a new holiday booking for you for your new travel date/destination.
If you decide to do this outside of 12 weeks before departure, we will ask you to pay admin fees of £35 per person on your original booking and to secure your new booking with brand new deposits. Your original booking will then be cancelled with loss of deposits and admin fees.
We'd like to say 'thank you' for staying with us by then giving you a discount on your new booking to the value of the original deposits you've paid (one deposit discounted per person still travelling with us). Please note that this discount can only be used by the customers on the original booking.
All of this can be done in one phone call to our Reservations Team who can handle everything from cancelling your old booking, to making your brand new booking and giving you that discount!
Please note: Changes of departure date or destination inside of 12 weeks before travel are subject to cancellation fees and the above terms will not apply - please call our Service Team for more details on 0844 557 4002.