Zipair charges 2,000 yen for a pax to change from one lie-flat seat to another during a transpacific flight, when all the seats cost the same to book initially. I would call that mean and customer-unfriendly, indeed.

I can't think of any other airlines whose flight crew would prohibit a passenger to switch to another empty seat after the plane has taken off (I'm talking about a like-for-like seat change, not a self-upgrade to an exit row, etc.). But I have no experience with LCCs.