Unindicated Switches of Instruments
There are some sections in a real orchestra that constantly switch between different instruments without you as a composer even noticing it.
The most prominent examples are the Clarinets and Trumpets. Even though a part might be written all the way through for Clarinet in Bb, you might have your clarinet players switch to Clarinet in A or keep alterning between Clarinet in Bb and A. This has mainly something to do with convenience as some key signatures are really awkward to play on the Bb clarinet and easier on the A.
With trumpets you might be getting switches between Trumpet in Bb, C and Piccolo Trumpet due to convenience reasons again. Some passages simply sit better on the C Trumpet than on the Bb trumpet and especially when there’s a part written for quite high trumpet that needs to be precise and defined, your (first) Trumpet player might switch to Piccolo Trumpet.
The decision for these switches should be left to the players and you should also not try to prevent them from switching as in the end they only do it to give you the best possible result. Also, you don’t need to worry about writing them another part for the alternative instruments. They will do the needed transposition in their head. Of course it is also possible to write specific parts for instance for Piccolo Trumpet if you are going for the specific tone colour but in a section, it will hardly be noticeable when switching to it.
Another example of a musician who switches instruments might be the tuba player who will switch between different models to create the best result for the passage written.