Change Instrument Mid-Staff

There are several different aspects to this. If your goal is to actually produce sheet music for people in a pit orchestra, then sure - just enter the notes, add a staff text to tell the players to change instruments, done. The trick is if for some reason you need the playback to reflect this change, and if you need it to be correct in both transposed and concert pitch mode. Then it becomes harder. In 2.0, you will be able to get the playback to change easily using the new Instrument Change element (found on the Text palette). But it still won't affect transposition. In 1.3, if you need the sound to change, you can edit instruments.xml, borrowing the tags used for trumpet to allow change between open and muted but modifying them to allow clarinet to change to flute or whatever. But again, no transposition change. Or, you can create both instruments on separate staves and use the "Hide Empty Staves" option, which will give you both the playback and transposition but won't work so well for the score - only the parts, and only if the changeovers always happen at system breaks.

So take your pick :-)