Can I renew my annual $300 plan with 16gb data?

Not unless you sign up as a new customer with a new number. 

This really (Edited per community guidelines) to be an existing customer!

Hi rogerlee850, we want to help with your Prepaid inquiry!

Let us know if you are inquiring about the 300/16GB plan. This offer is available for new activation and participating retail locations. Plan renews every 30 days for 12 renewal periods. The $300 advance payment is non-refundable in whole or in part. Next service payment due at 11:59 p.m. CT on the 360th day.

Learn how to make a AT&T Prepaid payment, you have options to

I had to port out and back in to get the $300 deal last year.  I guess I'll be doing the same thing again, because the only shortfall in the plan is the 8Gb data.  Just a suggestion...how about treating your existing customers as well as new ones?

My current 16GB att prepaid plan is about to expire on Dec. I see the new offer is worse than my current plan. If I want to keep my current plan next year, will just renew work? I don't want to be auto converted to the more recent offer. Based on the conservation above, it seems to be this case.

As long as you refill on time you will be able to keep your previous $300 one-year plan Just remember your plan is 360 days not 365.

Your expiration date of your refill and your renewal of your plan are going to be different The refill plan is going to be 5 days earlier than the expiration date.  You can add the $300 to your account before the date if you want.  Some people are taking advantage of target refill cards. Adding the refill cards just before the expiration of refill date. As long as they are $100 cards you're all set