While you can't change the size of the actual head tube (that is part of the frame) you can ask that the steering tube (part of the fork) be left longer- They are generally uncut when the shop gets them and cut to size when the bike is assembled. So it would have to be a new unassembled bike - once they are cut that's the end of it... (unless you want to replace the fork). Stems are easily interchangeable.