The /tʃ/ in the word "nature" is the result of palatalization (see this question). If I understand it correctly, the /t/ (nat) and and /j/ (ure) fused and produced /tʃ/. The letter U had the pronunciation /ju:/ (which is where the /j/ came from). So by palatalization, the pronunciation should be /ˈneɪ.tʃu:r/ (the vowel should not be affected) but now "nature" is /ˈneɪ.tʃər/ and the vowel has changed.
I searched the word's etymology in Wikitionary and according to that entry, "nature" is from Middle English nature and gives the pronunciation /naːˈtiu̯r/ and the entry also says Middle English natur: pronunciation /natuːr/, [naˈtˢuɐ̯ˀ] (I don't understand this square bracket transcription).
As seen, there is /u:/ in the pronunciation of "nature" but nowadays its pronunciation has /ə/ (UK) and /ɚ/ (US). There are many other words that end with "ture" and show the same change but I think this one is enough to demonstrate the change.
Was there a change from /u:/ to /ə/ (or /ɚ/) in the history of English?