One of the procedures to fit the head onto the block is to tighten the headbolts. These bolts are being stretched in this process. The bolts have been specifically designed to do so and are bascially designed to be used once.

When replacing a head gasket, you could ofcourse take the gamble and re use the bolts. They could be overstretched or they could simply snap while driving. Both will result in -at least- a blown headgasket.

If you don't want to risk the new head / gasket, you should buy a new set of bolts..