If you want to do a spring swap on the space bar, I would start by removing the Alt key caps either side of it. They should pop off quite easily - just make sure you apply equal pressure to both sides and pull straight up.
Once they're off, you can remove the space bar. The reason I recommended removing the Alt keys first is because the space bar has the main switch in the middle, and then two little supports joined by a metal bar. You need to pull the space bar off carefully and slowly, in order to avoid pulling off these supports. If they do come off, nothing is broken, but they're a pain in the arse to reassemble. Use small movements, gradually easily them off and they should stay attached.
Next, you need to open up the switch. I used two minature (i.e. those for repairing eye glasses) screwdrivers for this (one for unclipping and one to use as a wedge). Looking straight down at the switch from above, you should see that there are four little holes, one on each corner. These correspond to four little clips that hold the top piece unto the bottom.
What I did was use a very small screwdriver in the bottom left hole first to unclip it. The act of this should also unclip the top left clip, leaving the two left clips unclipped. Next, carefully wedge something thin underneath the left part, from the front, so it can't clip into place again and undo the top right clip and then the bottom right clip. The switch top should now just pop off.
Lift it off and put it somewhere safe. I swapped the stems and the springs so I can see which switch has the stiffer spring. The thing to take note of is that the Cherry logo on the black switch top should be at the 'top' along with the two little 'hooks' on the coloured slider (i.e. hooks + logo together).
I successfully opened 204 switches with this method and didn't break a single one. Maybe somebody else has a better method but this works for me, and once I got the knack of it I was fairly quick.
I would, however, recommend you start with the donor switch rather than your space bar. If you do break it, then at least you won't have killed your space bar.
Also make sure that your work area is clean, and that the keyboard is clean before you open the switches. Don't want hair and fluff getting inside.
Don't let my comments mislead you either. Cherry MX switches, although more resiliant to abuse than Alps, are still delicate things and going in poking around with a screwdriver is likely to break something. Somebody else may have some photos which illustrates how to do this better than my comments.