No, any phone manufactured before the new phones came out this year. I think that is any phone manufactured or offered for sale before March 2013. I may be wrong on the date, and others here will know the details.
I successfully changed my phone from the Optimus V to the Optimus Elite in May with no issues whatsoever and kept my $25 plan. Huge upgrade for me. Very happy with the Elite, and it's on sale now for $79. Amazing price for that phone. A real value. For my needs, it's all the phone I need, and more.
IMO, Virgin will not have a problem with you upgrading to another 3G phone, but a 4G upgrade may be an issue on a grandfathered plan. I'm not sure if there were any 4G phones before the manufacture Cutoff date. Again, others here will know. I'm speculating here.
Good luck!