Keep in mind that the changeover to Family Data won't be quite the same as the transition to tiered data was. Tiered data was a step backward for new customers, and not a good sign for the rest of us, but didn't really affect existing customers with smartphones. However, Family Data is actually good for new customers (compared to single line tiered), and may benefit a number of existing subscribers as well (just not those like us).
Given the ignorance of the average consumer, I would expect Verizon can make this sound appealing to new subscribers, which is where they expect to make most of their money. They really don't stand to make much extra from existing subscribers with this change, in fact a bit less. A family of 2-4 users each with a $30 data plan pays an extra $120 a month for their smartphones, and if none of them are heavy data users (which carriers insist most aren't), they will be able to save money by switching to a data "bucket" of between 6 and 10 gb's, which will almost certainly cost less than $120 a month.
So I would think that from Verizon's point of view, this switch will actually be attractive by itself to a number of customers, so the switch alone would be reason for a new advertising push. They could keep new subscription numbers high by advertising the S3 heavily now and after launch, then launch the Razr HD and attract even more customers, then switch to Family Data a few weeks later, "attracting" new customers with a shared data plan using one of the hot new phones they'll already have heard about. This is what I see as a likely scenario, launch the S3, Razr HD 2-3 weeks later, then go to Family Data late July/early August right before their Back 2 School promotions. Then launch a version of the One X as a follow-up to the Rezound soon after.
Of course that's all speculation, and the caveat here is the rather obvious fact that Verizon clearly wants to have as few subscribers on unlimited data as possible. If they truly wanted everyone to see this as beneficial, they wouldn't force people to change to it upon upgrading after a certain point. Whenever they switch, it will gain new subscribers, but makes existing subscribers not wish to renew after the date, allowing them to switch to other carriers. Having some great new phone options come out just before the date encourages a lot of existing subscribers to upgrade now and lock in at least another 2 years rather than going to another carrier, not to mention the rush of upgrades would look great on their quarterly statement.
I guess it all comes down to how much VZW really thinks they're losing on unlimited data subscribers, and whether they would rather have as few on unlimited as possible, or keep as many subscribers as they can for at least another 2 years.
Bottom line, they won't make a change without announcing the date first, even if its only 2 weeks notice, and I see no reason why we can't wait till then to preorder, or buy the phone if its out by then. Here's to hoping I can burn my 2 upgrades and have an SGS3 and a Razr HD before the switch!
