Here is what I understand from reading lots of news articles, posts on blogs like Droid Life, etc.
1. If you have unlimited data, you can keep unlimited data as long as you pay full retail for a new handset.
2. If you have unlimited data and want a subsidized upgrade, you have to give up unlimited. This means EITHER switch to the Share Everything plan, OR stay on your existing voice plan and add the tiered data feature. The plans as they exist today will remain available (except for unlimited data) for EXISTING subscribers. NEW subscribers MUST sign up for the Share Everything plan.
3. If you have unlimited data on your line in your family share plan, and another line in your in your family share plan wants to use a subsidized upgrade, they can provided they EITHER switch to Share Everything, OR switch to a tiered data feature (if they are currently on unlimited data). If they currently are on a tiered data plan, they can use a subsidized upgrade and nothing changes. The caveat is IF they DO CHOOSE to switch to Share Everything, then that means ALL of the lines in the family share MUST switch to Share Everything.
4. What gets fuzzier is if another line in your existing family share currently does not have a data plan (no smartphone) and switches to a smartphone after 6/28. Extrapolating from the information above (where the existing plans will remain for existing customers), then that means they can use a subsidized upgrade, but would have to add a tiered data plan (i.e. 2GB for $30, etc.). The assumption is that they would not be required to switch to a Share Everything plan. Again, the only customers that are forced to sign up for a Share Everything plan is NEW customers, since that will be the only plan available to them (except for individual lines which has the 700 minute plan as an option).
5. Also fuzzier is what happens if you add a new line to your existing family share plan after 6/28. Since adding a new line is not really joining Verizon as a new customer, then logic tells you that you should be able to add the line and add a tiered data plan, just like you can do today. Again this is because the Share Everything plan is an option to existing customers, not a requirement.