Mine

upper2lower() {
local THESTRING="${1}"

for letter in Aa Bb Cc Dd Ee Ff Gg Hh Ii Jj Kk Ll Mm Nn Oo Pp Qq Rr Ss Tt Uu Vv Ww Xx Yy Zz; do
    eval THESTRING='"${THESTRING//'${letter:0:1}'/'${letter:1:1}'}"'
done

echo "${THESTRING}"
}

lower2upper() {
local THESTRING="${1}"

for letter in Aa Bb Cc Dd Ee Ff Gg Hh Ii Jj Kk Ll Mm Nn Oo Pp Qq Rr Ss Tt Uu Vv Ww Xx Yy Zz; do
    eval THESTRING='"${THESTRING//'${letter:1:1}'/'${letter:0:1}'}"'
done

echo "${THESTRING}"  
}