Long story short you don't need to downgrade technically. What needs downgrading is the bootloader, also the 7.4.6 will sorta serve its purpose, you need to flash it for second d bootloader, however the current 7.4.6 freedomboot isn't compatible with 7.4.8 in the sense that it compares the freedom boot version with the os version and if they don't match it boot loops into recovery, which isn't a problem if you plan on flashing a custom ROM, so to make things easy just remember to put the ROMs you want to flash onto the kindle ahead of time or it will be slightly more difficult to do so once it starts boot looping into recovery. I recommend the fire flash method if you are not comfortable with a lot of command prompt commands, but in that method there's always the risk of you not seeing the downgrade bootloader option, its there, its just a lot of people tend to not see the check box next to or above the red warning text, and when its not checked you reboot with a nice red screen only a fastboot cable can fix. Just remember if you use the method you are doing, please for the sake of all that is good check the md5sum of the bootloader file you are going to flash to downgrade the bootloader, if you don't and its a bad download you will end up with a expensive paperweight.
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