Once a pathe has been converted to motion, you re-arrange the keyframe order to chenge thae staring point of the motion.
That's how I solved the problem, by reordering keyframes, and it works as planned. I was wondering if the graying out of the swiwtch has to do with imported paths.
If I may ask pose a different but related problem here, when I draw paths (not as masks but as shape layers) in AE, copy and then paste them into a property such as position, I get a new layer and not a motion path. I do have the shape path selected in the timeline, but still get a new layer. Pasting in an open path made in Illustrator works in the same comp, however.
This should work just the same regardless of the source of the path. You may just be selecting a wrong property level, so the data is not properly formatted and that may mess things up...