Having done something very similar recently I already had a script.
for f in */*/*
do
destdir=$(dirname "$f")
tvshow=${f%%/*}
season=$(dirname "${f#${tvshow}/}")
episode=$(basename "$f")
# Get season number
seasonnum=${season##* }
dest=$(printf "%s/%s - S%02iE%s" "$destdir" "$tvshow" $seasonnum "$episode")
echo "mv \"$f\" \"$dest\""
# mv "$f" "$dest"
done