Well, I,don't thing that sd card is important on,unlocking bootloader. The only sideffect of unlocking bootloader is the factory reset .Only your apps and settings are deleted. Your photos, videos and generally any file that is stored on your sd ( except apps) will remain untouched. If you are root you can titanium backup to restore your apps. Also make sure you have backuped your contacts.
Now your second question.
It doesn't matter what version you have. For example you can flash from cupcake to kitkat just like you would do if you was on ics or any other android version.
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