For many years, cuers were instructed that the spoken English vowel /iēeeiēeeiēee/ when unstressed changes to /ɪĭiɪĭiɪĭi/. According to this reasoning, the final syllable of the word happy was cued at the throat: /hhhhhhhhhæăaæăaæăaˈˈˈˈˈˈˈˈˈpppppppppɪĭiɪĭiɪĭi/. Instructors commonly told students that when words ending in unstressed /iēeeiēeeiēee/ (e.g., candy, cookie, trolley) are spoken naturally in sentences, the final vowel was more likely to resemble pit than Pete. After much debate and some division in the community, the consensus is that this pronunciation does not apply to many American English speakers. The word happy can be cued at the mouth.
There are many vowels that can be stressed or unstressed while not changing to another vowel. The word logo presents a decent example. The vowel in each syllable is the same – /oʊōohoʊōohoʊōoh/. But the stress is different. The first syllable contains a stressed /oʊōohoʊōohoʊōoh/ and the second syllable contained an unstressed /oʊōohoʊōohoʊōoh/. The two are differentiated by head thrust. When discussing a word like sleepy, instructors historically said that "unsressed /iēeeiēeeiēee/" should be cued as /ɪĭiɪĭiɪĭi/. However, the fact the they call it "unstressed /iēeeiēeeiēee/" instead of /ɪĭiɪĭiɪĭi/ is somewhat telling. If the vowel is truly just an unstressed version of /iēeeiēeeiēee/, it should be cued at the mouth without headthrust.
Unstressed Prefixes
This can happen. Unstressed prefixes especially can change /ɪĭiɪĭiɪĭi/ or /əəəəəəəəə/. Consider words like beneath. When the second syllable is stressed, the first syllable is likely to be pronounced as /bbbbbbbbbɪĭiɪĭiɪĭi/.
Dialect