Now I'm getting confused, Craft!
The first time I read through that stuff I read it one way, but on re-reading it I've interpreted it differently!
Having studied it further, it seems that if the RPI is 'hard-wired' into your existing agreement, HM Government had previously indicated that it would introduce legislation enabling pension providers to switch to using the CPI. But that page, dated only five days ago (and referring to an announcement in the House of Commons 'today') states that the decision has now been taken not to introduce such legislation. So (unless the terms of existing pension agreements allow otherwise) pension providers must continue to operate under their contractual agreement to use the RPI.
Chris