I sent an email to 1KVoice, and they handled it. It looks like they process this by redeeming the upgrade and depositing a new one. When I look at my account, I now see
2 Global Premier Upgrade(s) expiring 4/30/2014
6 Global Premier Upgrade(s) expiring 1/31/2015
But + 1's account shows
1 Global Premier Upgrade(s) expiring 4/30/2014
6 Global Premier Upgrade(s) expiring 1/31/2015
1 Global Premier Upgrade(s) expiring 4/30/2914
It'll be interesting to see if that GPU really does have 900 years of life in it...