Add pickup measure and move all other notes over

I copied a bunch of music from one file to another and somehow messed up the first measure, which is a pickup (1/2 measure) -- it is now a full measure. I know how to split it and change it from nominal to actual duration. But then I would have to split each measure in half and rejoin halves to get everything in the right spot. This is possible but tedious to do one measure at a time. Is there some way to move multiple measure over (if you understand what I mean) to fix this?