If you prefer using GNU Parallel the solution is:
find . -name "*[a-z][A-Z]*" -type f | parallel mv -v {} '{= s:([a-z])([A-Z])(?!.*/):$1 $2:g =}'
If you prefer using GNU Parallel the solution is:
find . -name "*[a-z][A-Z]*" -type f | parallel mv -v {} '{= s:([a-z])([A-Z])(?!.*/):$1 $2:g =}'