I've not had the letter, but figured I might do, so have been looking around at deals. The EE unlim calls+sms, 16GB data for £20 looks quite appealing!
My current deal with Three is 600 minutes (plus 300 3-3), unlimited texts, AYCE handset data and 4GB tethering for £18/month. The 12 month term on this ran out back in August 2015 so I'm just on a monthly rolling deal currently.
I called Three to get my Pac, they informed me that since my deal was not legacy it was not going to be cancelled at present - it would just merrily carry on. I'm not entirely convinced, my £18 deal was sold at the same time as the £12.90 200mins/unlim SMS/AYCE tariff which is being canned.
They did provide another offer though, which was Unlimited Calls, Unlimited SMS and 30GB of data to share between handset/tethering for £20 a month on a 12 month contract. It didn't take any real haggling, it was offered up almost straight away! Since my data usage is rarely over 10GB that would be more than ample for me.