Charges when changing from BBYW Opt 3 to Unlimited

[#1]

I am currently on the Broadband Your Way Option 3 and would like to change to the Unlimited package. When attempting to do this online it states that there is a one off charge of £104.27.  Why is this charge so much ? I have previously changed products in the past without additional charges.
As an existing customer, I am happy to start a 12 month contract to change but not the one off charge.

[#2]

Hiya,
If you give me the nod, I'm happy to sort out your account change without the cancellation charge.

[#3]

Thanks for the prompt reply. Would this be on a 12 month contract ? If so, yes, please go ahead and change my account.

[#4]

Thanks.
I've scheduled your account to change over on the 29th, which is your next billing date.
If you could possibly let me know once your account change has happened, I'll move you onto a 12 month contract.  I'm afraid I can't do that until the product change has actually happened.