I wouldn't advise changing just two tires, because soon enough the other two are going to wear out and it's a never ending cycle! Ideally you're better off replacing all four at the same time so they wear evenly and need replacement at the same time. Yes run-flats are expensive so if you can avoid them, I would. If you do damage one tire, you would have to replace it, and then you have one new tire and three worn out! So my advice...go out and but a good set of four tires, and keep a can of the flat repair kit with you!