It seems to me you simply either want to:
a) Automate the Tempo - Right Click on the Tempo and draw your curve. This will change the tempo of the whole song and if you have Delay lines set to Sync BPM then you can get "spang" sounds if the move is quick.
b) Select the Note Clip and Ctrl Click on the Drag Handles and re-size the clip to half the number of bars (eg. 8 bars to 4 bars). This leaves the song tempo alone but changes only the notes and automation selected.
c) F8 has a tool that lets you do the above but barely useful since you can drag
I think this solves your issue.