As I said, depends on the tyre type. If you put on some nice grippy front tyres, combined with half-used shitty Bridgestone Potenza RE040 standard rear-tyres, and you do some spirited driving you will have your tail out more than you want to (no, not THAT tail Sarnie ).
Then again, the Toyo's arent that expensive (imho), and for your piece of mind if you have doubts about it i would just go and get 4 new ones. Or get the fronts done first, find some abandonned parking lot and have some drifting fun, and get the back ones replaced shortly after. Better put the rubber that's left on the backs to some good use