The new scheme is basically saying passengers should buy at their first opportunity to do so, something that has already been publicised! One thing that will be changing though is places like Frome, that has a TVM and a ticket office (open part time) but is not currently included in the PF scheme - But will be in a couple of weeks.
The scheme would probably work better if they made more of an effort to install more ticket issuing facilities at stations that currently have little/none. There are very few on the Westbury - Weymouth line, none on the Severn Beach line, few at other Bristol area stations, none most of the time at Keynsham and Oldfield Park, none at Freshford or Avoncliff and obviously none at all GWR managed request stops. None at most Exeter surburban/rural route stations, and none at most stations on Cornish branches. And that's just the west!
Obviously you can't expect facilities at every station as the initial cost would be high and some lesser used stations don't really warrant it, but stations like Keynsham and Oldfield Park really should have better ticket issuing facilities.