Problem with tempo and tempo map.

I have a midi piece that has quite a lot of tempo changes. Unfortunately, I have decided that the entire piece is a bit too fast. So, how can I easily slow the whole thing down without having to redo the tempo changes? I tried selecting all of the points in the tempo map and them dragging them all down. That was a mess. The midi stretched out so that it would play slower, but the tempo map did not stretch out, so the changes are all in the wrong places and because the tempo map is no longer the same size as the midi files there is no way to line everything up. Is there really no way to do this other than completely redoing the tempo map from scratch?

There are actions which may help; search for tempo.... You can increase or decrease ALL markers by small amounts at a time.... As far as i can see larger changes (50%) are only possible on individual markers... I might be wrong though...

I used the playrate slider. I seems to slow down the speed at which the cursor moves. I have not tried to render yet to see if the slowed down playrate will render at the slower speed.