I just activated a "Samsung Mantra" from Virgin Mobile USA with another Sprint MVNO as a test. This was the phone that I used in high school and college, so it had been active on Virgin Mobile USA for several years before I replaced it with a new phone around 2011. That is well beyond the 1 year minimum that Sprint enforces following last month's rule change. Both Ting's ESN checker and their ESN checker said it was fine to activate before I did. I had to dial ##VIRGIN# (##847446#) to access a service menu to program it with the new number and MSID. It will oh accept a 10 digit MSID, so it is necessary to omit the first 5 digits, which should be 00000, when entering it. Afterward, it should just work. Mine did.
Old Virgin Mobile USA feature phones can be activated