Any way I could select 10 layers in my comp, then 10 new sources in the Project bin and swap them out?
Not that I know, since it doesn't provide a context to correlate the layers. There may be some script out there, so check AEnhancers.com and AEScripts.com. also remeber that you can use Ctrl+Alt+> to swap sources instead of Alt-dragging. this may already accelerate the workflow a bit.
One thought, which may be easier (or may be a lot of trouble) would be to move your original source material so AE can't find it, then in the find footage dialog select your new footage. This should work, but might get confusing.
Good luck, and let us know what you come up with for a solution!
Interesting ideas, Spontaneous.
Anyone have a suggestion on how the original project might have been built knowing ahead of time that 300 layers would need to be swapped out?
Thanks for the replies. Luckily the project changed a bit. So I only had 20 source movies to distribute to 400 layers.
I selected all my layers, then used Lloyd Alverez' Random Layer Selector multiple times. (Option-Dragging to mass replace randomly selected layers)
And then I selected all the layers and used his Layer Random Shifter to offset each layer in time.