Just swap the front with the rears if theres still at least 1-2mm of thread left. grip at the front is more important (breaking, turning) and also these will wear faster. so by the time they wear out u can change all four tires to non-runflats!
OR if u r desperate to change them, put the regular tires new ones on the back and the runflats on the front. With runflats on the front, they will wear faster and by the time they have worn out, the new tires at the rear would have been sufficiently run in. I dont think this would be too unsafe, as long as left and right are the same tire. But i wouldnt drive fast like this and just in case, u should change all four. Best idea is though...is to rotate them around like i said b4 and do burnouts for fun (video this and post it up!) and then change all four
