Not to be flip, but is this a trick question? When regular airfares decline, do airlines give PAX who have reserved at higher prices a rebate? I doubt they are more customer-friendly with respect to seat fees.
While I don't know about BA's policy, on United, purchases of EconomyPlus seating are completely nonrefundable. I think the only situation where you might receive a refund/credit is if you upfare your ticket to a higher cabin (e.g., Y to J or F).