You can walk from 2nd class to 1st class, no problem.
But do you plan on only buying 2nd class for Buchs - Salzburg? When bought sufficiently in advance 1st class on ÖBB is not expensive. With an SBB 1st class upgrade you can then travel all the way in 1st.
So you would then purchase:
- A 1st class ticket Buchs - Salzburg (without reservation)
- A 1st class upgrade Luzern - Buchs.
- A seat reservation for Zurich - Salzburg.