#!/bin/bash

find . -depth -type d -name '*_b*' -exec bash -c '
for d; do
    new="${d//_b/_a}"
    echo "DIR: $d -> $new"
    mv "$d" "$new"
done
' bash {} +

find . -type f -name '*_b*' -exec bash -c '
for f; do
    new="${f//_b/_a}"
    echo "FILE: $f -> $new"
    mv "$f" "$new"
done
' bash {} +