It is possible to increase the pitch even as you extend the slide

In general, the longer the tube, the lower the note. However, trombone players can play higher notes as they extend the slide, simply by closing their mouths and changing how they blow into the instrument. It's hard to explain in writing, so take a look at the video. This technique is used when playing thin sound patterns during jazz performances.

Video of a player extending the slide as the pitch increases