According to a Costco rep, if you pay the renewal fee ahead of the price jump, it would add on 12 months to your current membership expiration date. Just tried it. It says that it can take up to 24hrs to reflect on the account. Can report back if I remember.
CONFIRMED!
I had 2 months left before my membership was due to expire/renew. Now my online account says 14 months!
When paying for membership online, it only allows you to pay for one year at a time, and only if you're within 3 months of expiration.
It may be possible to buy several years ahead, if you do it before it reflects on your account. (I missed the chance to try it, since it's already reflecting on my account.)