You can put a jack under the engine, unscrew the fork caps and lower the bike so the spacer tube and spring rises up out of the forks. Taking them apart is not necessary to replace the springs or spacer and can be done right on the bike. In fact doing it this way you don't have to put then in a vise or risk scaring the finish anywhere.
Some will say that the oil needs to be dumped out and changed but if the bike is newer and the oil clean without leaking seals, why change it? If you want to just change to a different weight, a length of vacuum hose or something similar can be lowered into the forks and the oil sucked or siphoned out. If you are dealing with an older/high mileage bike ridden in dirty conditions, turning them upside down and cleaning out sediment would be advisable. But even in some of my really old bikes the oil was never dirty as long as the seals were good.