Clarification on JUMP! and T-Mobile Upgades in general

Last year, around April, my old phone finally quit out on me, a month before my contract ended. I used the dumb flip phone, and jumped ship to the prepaid $30 plan with unlimited texting and data on T-Mobile. I've been more than satisfied, as I rarely go above my 100m. The thing is though, how do my upgrades work? Am I able to sign up for jump, while keeping my $30? I'd love to ditch my ridiculous phone (unlocked lg quantum), as I'm a broke + desperate student, but I really can't swallow the $70 a month for unlimited everything, but $120 every year does seem pretty reasonable, if I can upgrade when I want.