Are o͞o and ü interchangeable?
Solution 1:
You should use neither of those for the phonemic representation of human. You should use the standard IPA, which is /ˈhjumən/.
So that vowel is /u/, or with the leading glide, /ju/. Sometimes the vowel is phonetically transliterated to [uː] but vowel length is not phonemic, at least in American dialects of English.