You can avoid dealing with physical discs, Thomas Rapp described it at MorphZone:
Updating procedure for...
Mac
You can also skip few steps in that guide. You don't need to rename bootinfo.txt or bless it (if you don't touch it). Unless bootinfo.txt has changed of course, but it usually (never?) does. It's enough just to replace boot.img and that's it.
This method has worked for me too, but it's not officially recommended to update the live system, because in theory it can cause some unexpected errors. I'd recommend to do it after fresh reboot at least.
Other safe discless method is to boot the ISO from USB memory. It's practically the same as CD boot and it's recommended way too. You can find official guide for it.
