I'm still looking at the price difference between individual tickets vs. the DB rail pass, as well as looking between 2nd and 1st class.

If I buy a 2nd class rail pass, would I be allowed to upgrade to 1st class on any trains? But choose not to upgrade on another day between two other cities?

For instance, if I'm railing between Basel and Salzburg, I may want to upgrade to 1st. But if I'm going Salzburg to Munich, I may just stay in 2nd. Is this possible?