Change the size of the scene, and move all the elements at once. .. How?

Hi.

I created a whole animation on a scene of a given lenght.

I want to reduce this lenght, without touching the scale of the objects, then move all the objects to recenter them (knowing that, unfortunately, the general anchor is at the top left and not in the middle.

The problem is that by selecting everything (either by selecting all layers with cmd-A, selecting all frames with cmd-A or by going to Edit > Timeline > Select all frames), everything seems to be selected but only the objects visible at the cursor location move...

How to proceed?