Again!? that will be the 3rd time this year!
If you are affected you will be told and then you will have 30 days to leave penalty free.
Same here. I'm in contract until March 2017 but speeds are starting to slow so want to get out before it gets too bad. (evening speeds down to 30Mb/s from 150)
I rang them but they couldn't tell me anything other than 'just wait for the letter'.
Based on the contract terms you have 30 days from when you get the notification (either by letter or e-mail). Or if you don't get notified then 30 days from the first bill with the increased charges. If you cancel within that time you won't have to pay the additional charges.
I'll be calling them again if I have not been notified by the end of the month.