Jayteeto.
It depends on how the organisation play the future!
At the moment, you are destined to stay on your existing T's and C's until
1. You leave or retire
2. You ask to go onto the NPAS pay scale (no guarantee that this would be permitted, but as it might be more simple to administrate then maybe the request would be approved).
3. Some time in the future the organisation decides that for an economic, technical or organisational reason, they need to put everyone onto a single NPAS pay scale.