That is exactly how Edge works you are signing a 2 year payment AGREEMENT and you will lose grandfathered features. There is NO way to keep it unless you add a new line and move the Edge device. Trust me I am in the same boat and I work for the company. Go look at the edge FAQ and you can call in and confirm from 10 different reps but we know you can generally get some incorrect information. So if you think myself and everyone who has the correct information is wrong try doing it yourself through online website if you are eligible for upgrade. Why do you think you need to be eligible for upgrade to sign this new agreement? Simply because it is an agreement while it is not a 2 year SERVICE contract you do lose unlimited data which is confirmed on multiple fronts.
Now I am sorry if you meant switching your plan as is going into Shared Everything but everything you said seem to be more so you wanted to protect your unlimited data. So the edge program works with all existing Phone plans you do NOT need to be Shared Everything but you will not keep unlimited data if you try to go directly onto the edge program from a phone that has the unlimited data feature.
Lastly almost all indirect and Corp stores need to activate the phone outside the holiday season because it is the way the store gets credit for the services they did for you. Generally not a requirement on full retail phones because there is no agreement but I can ensure you most stores still will enforce activation sadly