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But the mux doesn't have regional variations so how can it be switch of in one area?
All this seems to be is BT moving subscribers onto copper multicast where the facility is available - thus they get 3 sports channels rather than the two available via the encypted Freeview channels. It doesn't mean that the encrypted Freeview channels will cease to be transmitted at this time but rather that those affected users will no longer have active decryption cards and instead will get their BT Sports and ESPN channels via copper broadband.
No area will switch off.
What makes you think there will be?
I agree with StewM and zulu17.
Surely what BT are doing is identifying all customers who currently receive BT Sport over the aerial but are capable of receiving those channels via broadband Copper Multicast. They are then gradually moving those customers over to the multicast service, hence sending them a letter saying the change will happen for them on a particular date. They don't need to do this for all customers at the same time. Up to BT how they manage this change over.
Eventually the only customers left using the TV aerial transmissions will be a small number who can't receive the service via Copper or Fibre Multicast due to the limitations of their line (or a refusal to sign up to Infinity). At some point in the future for those remaining customers an unwelcome letter saying BT will be turning off the aerial service might appear, but for now the service is still being transmitted from all main DTT transmitters as far as we know.
It's not necessarily a lesser service.
As to why not move everyone over at the same time just before they turn off the DTT signals, well that would potentially be a recipe for disaster. They are trying to roll out the change in a controlled, well managed, staged manner. This is no different to how they are doing the BT Mail roll-out. Like it or not some customers can hit problems with multicast that need time and effort to sort out. In an extreme case they might even have to temporarily be put back on DTT. It also makes sense to build up the extra demand on the broadband back-end systems gradually and monitor performance. To cross their fingers and just hope they could move everyone across one weekend and it would all work flawlessly on the Monday would just be foolhardy. Imagine the deluge of complaints they might get from people who could no longer access the sports channels for one reason or another.