To switch to 24V you should get a new bed heater, new hot end heater cartridge, and new fans. I don't advise running a 12V heater on 24V at low PWM because of the fire risk if anything goes wrong. It's possible to add a 24V to 12V converter so that you can go on using 12V cartridge heaters and fans, but cartridge heaters and fans are so cheap that I don't think it's worth the hassle.