I have a phrase consisting of 8 quavers (1 beat note?) over 2 bars (ie.4/4 time). What I want to do is to select those notes, then change them all so that I end up with one bar containing 8 semiquavers (1/2 beat not?)  (still 4/4 time) - and for preference in Staff View.

In the PRV:
 
- Enable Snap at 1/4 or a Measure (use Snap 'By' if notes aren't quantized to beats).
- Lasso the two bars of notes (right-click and drag across the time time and pitch range)
- Ctrl+click near the end of the last note and drag it back to end of the previous bar.
- Select and drag everything later back to fill the empty measure if necessary.