To take the springs off you need to remove the struts and compress the spring before undoing the retaining nut on top - IF you don't compress teh spring be prepared for the strut top (spring retainer and nut) to smack you in the mouth.
After the spring is compressed it is straight forward - when installing new springs, they are sometimes not very compressed at the installed height leaving you to fit the new springs without having compressed them first.
Whether or not you use the existing shocks is a matter of how good their condition is - remember that the shock is there to control the spring, and uprated springs will usually required shocks uprated accordingly.