GO Contract: Can they simply change their contract?

Just received an email from GO informing me that they are increasing their Internet speeds but also increasing the cost by €2 a month mid-contract.

Searching through the contract there is a proviso that they can change and amend offers and costs, but is this legal after all?

And to preempt some comments, its not because of the €2, but the fact that they're changing the contract midway, whilst I get a penalty if I try to terminate it early.