OK, so no partner induced prebend. So mast prebend is due to interaction between uppers (inducing bend) and inters & lowers (removing bend)
What is this "winter relax" thing?
Without knowing your exact set-up: easing the h/s will only remove some tension on all the aft swept rigging (uppers, inters, lowers). But at the same time, if mast partners are firmly chocked, it will increase the pre-bend they induce.
Generally in your situation, to reduce pre-bend one would...