I've not seen this letter or anything, but my understanding is that once you have completed the total number of years required to get a full pension, you can retire. FULL STOP.
Ie the police pension scheme, you can in theory retire at 48.5 years old, as long as you joined when you were 18.5... there is no way of changing that. - Unless they change the pension scheme, like they have for the police, but only for new employees...
James