Titleist does not sell heads only. Beware of any on-line sale for heads. If they were not pulled from existing shafts, they are likely counterfeit. Nothing wrong with reshafting the irons you have but you might find the price deferential to be minimal of just selling your old set and applying the money to the new T-series with the shafts that fit best. Do a Titleist Thursday or similar fitting to pick the head/shaft combo that works best for you. Good luck.

Whether you are playing X, S or R shafts, you need to be on a launch monitor to see what your swing speed does with the different shafts. Despite all the rhetoric, I think most people play shafts that are too soft for their swings. Not swing speed, exactly, but how the transition occurs.