Just so everyone knows, Virgin Mobile is going to discontinue the old plans ($25/month) for any new smartphones that you swap with by May 27th. This also includes those of us that are grandfathered in.
If you do not plan on upgrading to a different phone with Virgin Mobile, then you will keep the $25/month plan until you discontinue the plan or switch to a new phone after May 27th.
I think that Virgin Mobile will probably be pushing out their new phones (HTC One V, Evo 3D, or whichever phone they do release...) at around June. I am only basing this off their change in grandfathered plans.
Source
TL/DR: If you have a grandfathered plan ($25/month), you keep it (unless you upgrade to a newer phone after May 27).
Edit: You can upgrade to any phone that was released before May 27th and still keep the grandfathered plan of $25/month. But any phone released after May 27th, will get you on their new priced plans ($35/month).