Talked to multiple Verizon reps on this issue. I ordered the S3 before the 28th but was waiting for the Inc4G (my 4.3" phone is already too big).
I was told by multiple reps that as long as you exchange it in a Verizon corporate store during your 14 day trial, you will keep your unlimited data. You cannot exchange it at any authorized retailer or send it back to Verizon, since they'd need to inspect it and they'd then reset your upgrade, but since it's now after the 28th, you'd have to give up unlimited. But apparently exchanging it at a corporate store is ok since the employees can inspect it on site and enter it into the system at the time of exchange.
The only thing that could screw you is if you didn't accept the terms & conditions when you ordered the phone. If you ordered over the phone after it was done they placed you on hold and then you had to confirm you were ordering the phone and extending your contract by pressing "1" for yes or "2" for no. I believe they have something similar if you ordered online. But if you didn't you'd be losing your unlimited anyway since you'd be acctepting the t&c after the 28th, so probably not a worry.
One other thing, have you already opened the retail box (the one that says Samsung on it)? If not, they can also waive the restocking fee since the box hasn't been opened and they can sell it new.
As I said, I've talked with multiple reps (around 6 CSRs & 4 store employees) who've confirmed this. I check back often to see if they change their story. Normally if you just ask they'll say you'll lose your unlimited, but when I tell them what i have been told previously, they'll confirm I am correct. Basically it means they'll take your unlimited if you don't know that they don't have to, but if you do know you should be able to keep it.