Perl 6, 32 bytes

*>>.&{(try $_-1)||.succ.comb[0]}

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Perl 6, 31 bytes

*>>.&{(try $_-1)||chr ord ++$_}

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Anonymous Whatever lambda that maps each element to the list and attempts to subtract one from it, otherwise incrementing it and taking the first character in the case that z wraps over to aa.