What Happened to Old English Vowels?

Early English had single syllable nouns which had a back vowel, either ah or u. In order to make all of those words plural, old English used a suffix, e's, spelled i-z. So the plural version of mon, monies, became minis. And over time, that suffix,e's, was dropped. And the result was min and min was left as the plural version. But here's the key to all of this. As English Yvonne very early on, in the first century or so that the Anglo-Saxons were in Britain, the pronunciation of that first syllable started to change. It was the product of that natural laziness