I am currently running Android L right now using multirom and haven't had to do any factory reset.
Here's what I did: I downloaded the flashable zip for Android L. Booted into my recovery, went to multirom, added the .zip for Android L and installed it, set it up and I still have my other main ROM (Graviton) to boot into.
*EDIT* or you could go old school and do a full back up of PA and then install Android L, see what you think and then restore PA after you've checked out Android L. Best to do the multirom method though as there's no fussing about
So no factory data reset with this method.
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