Upgrade from Ultrafast Broadband to Full Fibre

[RDR]

I currently have Ultrafast Broadband and am thinking of upgrading to Full Fibre.

Will I be able to upgrade on my current contract or would I have to take out a new 24 month contract?

Also (like many others) I have been offered a better Broadband deal through my mobile phone provider, but have a few months left on my Sky contract. What sort of deal does Sky offer existing customers, as their deals to new customers seem competitive.