You can command-select the layers you want to move and drag them forward or backward in time. It's easy. There is no way that I know of to do this automatically if you stretch a layer. AE does not have insert-and-ripple capabilities like a video editor does.
Place or move a layer in time
The layer duration bar represents the layer duration visually. The In, Out, and Duration columns in the Timeline panel represent the layer duration numerically.
To choose which columns are visible in the Timeline panel, choose Columns from the panel menu, or right-click (Windows) or Control-click (Mac OS) a column heading.
These procedures move the entire layer in time.
- To set the In point or Out point numerically, click the number in the In or Out column for the layer in the Timeline panel.
- To move the In point or Out point to the current time, Alt-click (Windows) or Option-click (Mac OS) the number in the In or Out column for the layer in the Timeline panel.
- To move the In points of selected layers to the beginning of the composition, press Alt+Home (Windows) or Option+Home (Mac OS).
- To move the Out points of selected layers to the end of the composition, press Alt+End (Windows) or Option+End (Mac OS).
- To move selected layers one frame later, press Alt+Page Down (Windows) or Option+Page Down (Mac OS). To move selected layers 10 frames later, press Alt+Shift+Page Down (Windows) or Option+Shift+Page Down (Mac OS).
- To move selected layers one frame earlier, press Alt+Page Up (Windows) or Option+Page Up (Mac OS). To move selected layers 10 frames earlier, press Alt+Shift+Page Up (Windows) or Option+Shift+Page Up (Mac OS).
- To move the entire layer in time by dragging, drag the layer duration bar to the left or right. To snap the layer duration bar to significant points in time (such as markers, or the start or end of the composition), Shift-drag the layer duration bar.
When you drag a layer in the Timeline panel, the Info panel displays the name, duration, change in time, and In and Out points for the layer.Before and after dragging the duration bar
Jeff Almasol provides a script on his redefinery website with which you can move selected layers as a group, aligning the group to a specific time in the composition.