I added an offer on Sky sports to my package. On the checkout page, there was an icon that confirmed any addon to the package can be cancelled with 30 days notice.
There doesn't appear to be any reference to this on the order conf. Live chat does seem to be able to confirm. I thought all addons were 30 day cancellation (as it said that on the upgrade page in a couple of place), but need to check.
Stand alone add-ons require 30 days cancellation notice, they don't affect your contract length or any current discounts you may have.
Bundle changes which add or remove Sky Sports tie you into a new contract.