You change your PE ticket, even if you downgrade, so the costs is accordingly to the PE price.

Your options are (if you really want to downgrade):

Pay the difference
Hope AY is changing some of the flight times so you can cancel/change for free
Move the PE ticket to another date and fly on your own (for holidays i.e) and buy the Y ticket for the needed dates.

If you do the last, you have more chances that AY will touch the times and you can cancel for free.