Hi Peppe,
Do you mean that you've yet to take up duty in the new post? I've no experience of working in the rail industry, but if that is the case, I'd not expect to be paid at the higher grade until I took up the new (higher graded) post.
Hopefully someone in Scotrail with knowledge of how appointment to a higher graded post while still performing the duties of the original post will come along and help.