This is completely stupid !
If customers dont want fibre because 10mbps is enough then switch them to fibre for free and limit the bandwidth to 10mbps, what is so difficult with that !
Ah, they want the customer to pay for something they dont need or want at the moment - I bet they do, well that isn't going to happen and saying they will end copper at a specific date will depending how they do it will potentially mean some people will opt out of internet services due to the added expense of a service they don't want or require. Just reduce fibre packages to ADSL prices with a free bonus speed upgrade from 10mbps to 20mbps or 30mbps.
It will not go down well if you say look buddy your £10 a month unlimited ADSL at 10mbps is being discontinued so upgrade to fibre at 50mbps at £50 or lose internet services ! I know I would be pissed even if i was out of contract !
Its much better to use a carrot than a stick so do packages that make it enticing enough to make the jump to fibre or upgrade them as I said for free but limit their bandwidth to 10mbps or better still upgrade them for free to 30mbps and dont tell them what you have done !