I sent a message via Twitter to @askamex about upgrading, and prompted me to send a direct message via my account on amex's website. I did, and a few minutes later I was shown this exact message, word for word:
At this time you can change your Zync Card to an American Express Green Card which has a $95.00 annual membership fee, an American Express Gold Card which has a $125.00 annual membership fee, a Premier Rewards Gold Card which has a $175.00 annual membership fee or a Platinum Card which has a $450.00 annual membership fee.
To compare Card benefits, please visit: americanexpress.com/apply
Please do not hesitate to respond if you need further assistance or have any additional questions. It is my pleasure to assist you.
Social Media Response Team"
So by this, or at least until I actually do a PC to another charge card, it's safe to say you can upgrade. I had the green at one point, but cancelled it since I was working on other signup bonuses for most of the last couple years. I'm considering a Gold since I'm looking back into travel again, but the AF of the Zync is still pretty good for what I use it for anyway.
Thoughts? Are you guys looking into upgrading as well, or staying in the Zync realm for awhile?