No. Installing SoftStep 2 will not overwrite the old editor. The act of "installing" is really just copying a folder onto your hard drive. The SoftStep 1.21 and SoftStep 2 folders are named differently and do not affect one another — they exist separately and both can still be used. You'll have to change firmwares when switching back and forth, but it will still work (note that SoftStep 2 hardware won't work with the old 1.21 editor).