Here are two place names whose pronunciation history converged for a time and then diverged again.
Wymondham is, in my experience, always pronounced /ˈwɪndəm/. I don’t have access to either of the Engish Place Name Society (EPNS)’s volumes on Norfolk, but the Oxford Names Companion gives its origin as ‘homestead of a man called Wīgmund’. The pronunciation was presumably something like /ˈwiːgmʊndhæm/. Over time, the /g/ and /h/ will have disappeared, the /ʊ/ and /æ/, being unstressed vowels, will have reduced to schwa, leaving /ˈwiːməndəm/. The first schwa then also goes the way of all flesh, and the /m/ assimilates to the following /n/: /ˈwiːndəm/. At some stage /iː/ is shortened, and hey presto, we have /ˈwɪndəm/. This must all have happened before the Great Vowel Shift started to apply.
Wymondley, according to both the EPNS on Hertfordshire and the Oxford Names Companion, has a slightly different man’s name as its root: Wilmund. The EPNS volume was published in the 1930s, and gives the pronunciation, as we might expect, /ˈwɪmli/, following the same sort of path as Wymondham. However, the only pronunciation I ever hear now (and I live about three miles from Great Wymondley – Little Wymondley is a couple of miles further away), is /ˈwaɪməndli/. I’ve just consulted a scion of a long-established Hertfordshire family, and she tells me her father used to say /ˈwɪmli/, but only in a jokey sort of way. So, the spelling pronunciation was around for most of the 20th century, and can’t be attributed simply to incomers to Letchworth Garden City (founded in 1903) and Stevenage New Town (1947). This distorts the etymology: there was never a long vowel in the first syllable, so /ˈwiːməndli/ was never the pronunciation, so far as we can tell, and /ˈwaɪməndli/’s only justification is as a spelling pronunciation.