A change of hours affects any benefits you build up from 1 April 2015 from the date of the change.
A change of hours does not affect any benefits built up before 1 April 2015.
A change of pay affects all benefits.
If your pay is being reduced through no fault of your own you may be eligible for a certificate of protection. Appendix 2 of the HR Guide in LGPS Guidance for Payroll / HR practitioners explains how certificates of protection work.