I really don't think you can kill forsythia. Being an avid "plant-mover" myself, I do it all the time until I get things where I want them (I call it the "sink or swim" method). With some more sensitive or non-hardy plants, I would think twice about what time of year to move them, but with forsythia ~ go. It may look a little stressed for awhile, but it has all spring, summer, and into fall to get settled in.
ETA ~ do not prune it again until after it blooms next spring (in case you didn't already know that:)