There is no such thing as "grandfatering" unless it is specifically within the contract. A month to month agreement is not only at your option. If it's truly month to month than, unless otherwise stated in the agreement, Verizon Wireless can terminate that agreement and tell you that the only month to month agreements it offers are the current offers. It's an unsettling situation considering that there is virtually no ability to move to another carrier with your phone to obtain different service.
While that is true, Verizon has historically grandfathered people's plans and features. For example, while for certain phones (i.e. Voyager) Verizon did require people to switch to the newer Nationwide plans and lose their older America's Choice plans, for others (i.e. smartphones) you could keep AC and just add the smartphone data plan.
Also, Verizon offered the Talk, Text and Data promotion where you got $20 off of each line with unlimited data, but did away with the promotion a couple of months ago. Although the promotion is no longer available, it did not drop off for the people who got it. And for people (like myself) that did a plan change (changed from one unlimited data plan to a different unlimited data plan) which triggered the promotion to automatically drop, it was a simple matter of calling in to have the promotion (even though no longer available) added back.
So making a blanket statement that says because there is no specific wording in the contract that says Verizon will grandfather does not necessarily mean that they won't.