ISO files are still useful, for example to test in a quicky way on a Virtual machine, but I don't know you, but I'm sick of burning CDs....
I think that the best and more confortable method to upgrade the OS should be:
1. Download a LiveCD Fedora.iso (it takes less time than download the DVD iso file)
2. Launch fedup --iso Fedora-LiveCD.iso, that make the job of adjusting GRUB2 i.a.
3. Reboot the system
3. Choose LiveCD entry from GRUB2) Perform the installation as you wish
And forget about of burning optical discs....
Of course this does not cover all cases, for example, you should preserve the partition where ISO image file is.
What do you think?
Greetings