Since Verizon has not officially said how this will all work, EVERYTHING is speculation at this point. Yes, we HOPE that we will stay grandfathered in even if we upgrade, renew, or switch to 4G, but there is currently no official confirmation. There ARE, however, different degrees of likelihood, in my opinion (which admittedly is just more speculation):
1) It's probably safe to assume there will be SOME kind of grandfathering. Obviously, if you are still under contract and stay with the same phone, you will keep Unlimited data. CERTAIN.
2) More than likely, if your contract runs out but you stay with the same phone, you will be able to keep the Unlimited data. VERY LIKELY.
3) It also appears likely that you will be able to keep Unlimited data if you upgrade a 3G phone to another 3G phone. This will be in line with what AT&T does. FAIRLY LIKELY.
4) What's NOT so clear is whether you will keep Unlimited data if you upgrade from a 3G phone to a 4G phone. Since the plans are priced the same, there's a chance will might be able to keep it. Then again, Verizon's track record lends credence to the belief that they may force you to switch to tiered, ESPECIALLY if the new iPhone is LTE (which is likely is not). UNCERTAIN.
5) Another uncertainty is HOW LONG they will let you keep Unlimited? They may let us stay Unlimited for one upgrade/renewal, but let's say a 4G iPhone comes out NEXT year. Will they still let you keep Unlimited? More than likely, you will eventually be forced to go Tiered at some point...
6) And finally, do we know if Tethering will affect this in any way? For example, on AT&T you MUST switch to tiered if you want Tethering. On Verizon, it APPEARS that you can add/remove the Personal Hotspot feature without modifying your plan, but that could possibly change after the tiered pricing goes into affect also. UNCERTAIN.
The question is whether Verizon will make all of this clear BEFORE the official changes take place?