First of all, re-shafting clubs can be expensive - not quite as expensive as a brand new set, but when you factor in the cost of the shaft, labor and new grips you can easily be looking at about $100 per club. (Give or take depending on the shaft) being that your irons are 8 years old - even in good condition the grooves will wear down especially on forged clubs, you may not notice it, but your regular 160 club might be going 156 if the grooves are worn. On a new set Titleist will also calibrate the swing weight to match the shaft - i assume you want something lighter than a dg s300 you may even want to try graphite. A certified club fitter can help you select what may work best, but it may be best to consider going for a current in line set especially if the cost isnt much different, if you trade in your old set it might be a wash to just upgrade to a current model.

Even if it isn't a $$$ wash, there won't be a 912 fitting head to trial the feel of the head with the available shafts for an optimal fit. The T-line will be out for another 18 months, so you can replace 4 of the irons now and 4 later and have a set of irons that will fit you much better in 8 years than your 712's will with new shafts that may or may not be your best. Besides, you may find the T-200 to be better for you now that an AP2/T100. Worth more than saving $50-70 per club and needing a whole new set in 4 years.