As of now, you can't unless you update to 3.7+ OFW, at which point you lose all your 3.55 CFW goodies
There are ways of dual boot firmwares, using e-flasher, but checking the Ps3 boards, nobody seems too fond of the device yet. Seems that they have a few kinks to work out. If they do as they originally promised (full dual firmware boot) you can have the best of both worlds (CWF/OFW). Keep your fingers crossed.