Okay, after talking with roughly 10 CSRs over the past week, and checking the forums at Verizon wireless, this is what I've got from them.
Anyone ordering/preordering a phone before the 28th, and accepting the terms & conditions before the 28th, keeps their current plan. (We already knew that)
You may exchange to another phone after the 28th if you're still inside the 14 day trial period and keep your unlimited/current plan. But if you preordered through online or Telesales (like I did) and want to exchange, you must go to a Verizon corporate store to do it (no third party or authorized resellers). I was thinking they'd send out the new one and I'd ship old one back in the box the new one came in (like with a warranty return). But for a new phone exchange, Telesales would need the old phone first to check it out, which would essentially be a return. That normally wouldn't be an issue, but it is with the whole 28th thing going on. But, if you take the old phone into the corporate Verizon store, they can inspect it, enter it into the system & exchange it in the same visit, and keep your unlimited data plan. This was confirmed by the CSR, her supervisor, and the manager at my nearest corporate store. So if you have preordered the SGS3 and written off being able to get the Inc4G, this is the way to do it. Since it is launching on July 5th and the SGS3's are shipping a few days later, there shouldn't be an issue exchanging since the Inc4G will be out before we'd receive our SGS3's.
If you receive the SGS3 (or any phone) and are thinking about exchanging it, you can also eliminate the $35 restocking fee. If the customer either refuses the shipment or doesn't open the retail box, the restocking fee can be rescinded. This is per the CSRs at *611. If you're planning to do an in store exchange, I was informed that they still have to charge the fee, but a CSR can refund it. So if you're planning to exchange your SGS3, don't open the retail packaging.
If one of the people on a Family Share plan upgrade with subsidized pricing, they will lose their unlimited data, but they won't kick everyone else on the plan to a Shared Data plan. The one upgrading will get a plan just like they currently would, a minute plan with or without text and tiered data at the pricing they currently have (2GB for $30). They may also bring back the double data promos once in awhile also. This is why they've been saying they won't be forcing people to Shared Data. Basically if you choose you can have a tiered plan like what's been available for the last year.
P.S. Going back to the exchanging phones after the 28th, I also have screenshots from the Verizon forums with a reply from someone with Verizon Wireless Customer Support (Twitter@VZWSupport) stating that "Our 14 day return/exchange period will allow you to keep the existing plan if you decide to exchange after the 28th of June. If you do upgrade after June 28th, then the line will migrate to tiered data. Thank you!" I would post it but am not sure how to post pics on here (did it once but forgot how to). It also has the support guy's name so I wasn't sure if that would be koscher to post it here.