Is it reasonable to expect that an experienced cornet player should be able to pick up a "middle of the band" instrument, like a baritone, or an Eb horn and become reasonably proficient in a few hours across a day or two? 

How large is the shock to the embouchure and how costly is it to then move back?


Question: Ad layout for programs - Illustrator or InDesign
Answer: Depends entirely upon ad style.

If the ads are more illustrative in nature, I'd use Illustrator for the ad, then place then in a master Indesign document. Ads with text transforms, image interactions (cutouts) or with other non-critical design elements such as spot illustrations or flourishes can often be easier to create in Illustrator.

If the ads are more text-based, I'd use Indesign for the ads as well as a master document.

There is no set rule as to which one is better or should be used - Illustrator or Indesign. It really all comes down to personal preference and features needed.