Thanks! Will check it out!
If you want to change all line caps, even in multiple layers type cap in the search field in the top of the timeline, draw a selection around them all, then change 1 and they all change.
Step 1:

Takes about 5 seconds, then step 2:

The result:

This works for any property in the timeline. If you want only a few properties to change at the same time hold down the Ctrl/Cmnd key and just select the ones you want to change:

No script needed. If you had a glow added to 20 layers you could change the Color B of every glow using the same technique:

This technique is in the user guide.