Is "woman/women" the only English word group that undergoes a change - via conjugation, pluralization... whatever may be the cause - wherein a syllable of the polysyllabic word has spelling changes that do not have a significant change to its pronunciation (i.e. the "-man" and "-men" sound as "mun" in both cases), while simultaneously having another syllable that Does have a change in pronunciation, even though its spelling did not change (i.e. the "wom" pronounced as "woom" (or womb) and "wihm" (wim, whim) respectively)?
Mind you, "children" is not a correct answer because it begins as a (albeit, differently pronounced) single syllable/word and transforms into a polysyllabic word.
Edit: to clarify, I'm looking for specific words that follow the unusual pattern of the "woman/women" construct: a specific syllable changes pronunciation without a spelling change while another syllable changes spelling without a pronunciation change. One commenter @john Lawler pointed out the Umlaut factor in this word and sparked a notion to me that the answer might lie among German cognates, but in my life I've never found any words like this and that is why I ask the question. If you have evidence that this word group is not unique for this quality then please provide examples. Also, my curiosity is so piqued by Lawler's suggestion that I would welcome even German words that do this as examples, though I'd request a translation and phonetic transcription with that, please.