Yes, this is what the ticket office agent told me, when I redeemed my upgrade vouchers for traveling during next membership year, if I need to change the dates after the membership year, I won't lose upgrades without having to use another voucher(s) during the following membership year. Watch your ticket's date validity, some promo Y fares have very limited dates.

Technically, once your ticket is upgraded, you have a J ticket so as long as you pay a change fee and the new dates have J fare buckets available. I do need to caveat not all agents are knowledgeable (some just don't want to deal with it) so if you hear different answers try somewhere else.