The policy change follows a case at 1 Curzon Road, I believe. The owner of a block of flats installed upvc windows without consent and CCBC requested he replace them with hardwood, as it was in a conservation area. He appealed to WG and they upheld the Council's decision. As such, CCBC now have the mandate to carry out similar enforcement actions in Conservation areas.
I think a compromise would be the best way forward. No more upvc windows to be installed and a requirement that owners replace any existing upvc windows with hardwood when the time comes to replace them.