I don't see how slowing the FPS will help you. A quick option is nest/precomp your timeline and apply Time Warp with the appropriate settings.
If you want to be precise and extend the duration of your comp and layers then that's actually not too difficult too. After lengthening the comp and layers. You can Time Stretch your KFs. You can do this quickly if you reveal each layer's KFs ("u" shortcut on selected layers). Then CTRL+ALT+A (which selects all visible KFs). Then ALT+drag out the last KF until it reaches the new end - all other KFs on other layers will Time Stretch accordingly.
One weird thing to note is CTRL+Alt+A literally selects visible KFs - ie KFs that your eyes can see. IOW, if you have KFs or layers not in eyeball view, then these will not be selected.
Depending on the number of layers you have in your comp, it's really fast and easy to change your duration.
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Great tips from Roland! Just be aware that you may need to tweak your keyframe interpolation if you do this as it will affect the velocity and speed between keyframes. Use the Graph Editor (and the info panel for reference) to do this.
You can also use the Free Transform feature in the Graph Editor to retime groups of selected keyframes. Sadly this doesn't work across multiple layers but works over multiple properties.
Hope this helps!
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