Try calling and asking if the name of the account holder changes will it lose the grandfathering. If they say yes, then if they find out that it isn't you anymore they will switch it.
TL/DR - VM doesn't care who we are, they just care that top-up money shows up in the account every month. VM never verified your identity up front, so they have no way to prove anything about anything related to a subscriber's identity.
Calling them is a reasonable suggestion, but my concern is that we could call VM customer service ten times and we will probably get ten different answers (about anything, not just this). I definitely would not consider the statement of a VM customer service representative to be authoritative on this matter unless they can give you the name of the document that specifies that rule or policy, what the relevant language is and where it is published. Even if it's not published publicly, you should ask them to read the relevant language from the internally published document (because they'd never just start spouting off some random answer from "memory", right?
).
If they can't answer the pretty simple and straightforward questions above, you should file a complaint with your state's Attorney General's Office of Consumer Protection. When companies try to enforce secret rules, that usually doesn't go over too well with the AG's staff.
This goes to the fundamental relationship between company and customer, so you'd think that if it was important to VM, change of control would be covered in the Terms and Conditions. However, the T&C's say nothing about change/transfer of control. The FAQ are also silent on the subject.
Some things to think about -
If a woman gets married, changes her name, then updates her account with her new name, does she lose her grandfathered rate? Since that negative consequence would disproportionately affect women, would VM USA be guilty of enforcing a discriminatory practice? Do you think the Sprint Nextel legal team would approve something so obviously unfair to a protected class?
Remember when you signed up for service and VM made you send in photocopies of your official, state-issued ID proving that you are who you claim to be? You know, so VM could verify your identity? Because the identity of their subscribers is important to them? (Kidding! They don't do any of that!) They don't know who we are because they never asked. Because they never verified our identity up front, there's no way for them to prove that a name change also represents a change in the person who "owns" the account. Even if the policy did say "You can't do that!" there's still no way for them to enforce it since they never verified our identity in the first place.
As long as top-up money shows up in the account every month, they really don't care who you are. Even if they wanted to care, the way their system is set up now, they have no way to prove who you are anyway, before or after a name change on your account.