Yes, upgrades should work fine (I hope ).
Just tried this (not all the way through to payment).
I put the GDNet+ Standard in the shopping basket, and went to checkout.
The price listed (for yearly) was $64.99.
Basically, I had expected ~$25.00.
Logic I'm following:
I have a running GDNet+ Basic subscription (cost = $39.99).
I want to upgrade to GDNet+ Standard subscription (cost = $64.99).
What I should pay for upgrading = $64.99 - $39.99 = $25.00.
(This ignores the fact that the subscription I have is probably mid-way through or something, but it shows the logic I'm following).
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Another bug:
http://www.gamedev.net/page/subscribe/index.html
I selected Yearly.
Then I selected Sign up on one of the items.
Then I hit back in the browser.
Result: Yearly is still selected, but the prices are for monthly.
Would expect either Yearly to be selected and prices to reflect yearly prices, OR monthly to be selected and prices to reflect monthly (first option of Yearly + Yearly probably what I would expect the most out of the two options).