can you replace steel shafts & put graphite in instead, a golf retail person said no as it would upset the balance, is this correct or are they just trying to flog me new irons
The swingweight (and deadweight) would be radically altered, unless you were replacing a lightweight steel with a graphite of similar weight. In which case there would be little point.
In the hands of an expert fitter, and with plenty of money, it may be possible, by adding lots of weight into the head in the correct positions, but buying a new set is probably a better bet.