I moved from the old Entertanment Max HD package to the new Big Entertanment package on a Friday evening without to many problems. I also canceled BT Sport as that's now not included in both Entertanment packages. I also added the HD boost to my package.
It tane an hour or two before I could activate the Now TV app on my Ipad and was not till the Saturday that my sister could use the Now TV app on her Samsung smart tv.
They was going to send me out a new BT Youview box guy managed to cancel sending new box asi said I don't need a new box. He even managed to stop the set up fee for the new package.
so overall I have not had many problems with the switch to the now package.