The upshot has been that the Anglo-Saxons lived (like the Scottish still do) in a 'hoose', and the English live in a 'house'; the Anglo-Saxons (like the Scottish) milked a 'coo', and the English milk a 'cow'; an Anglo-Saxon had a 'gode' day and the English have a 'good' one; an Anglo-Saxon had 'feef' fingers on each hand and the English have 'five'; they wore 'boats' on their 'fate' while the English wear 'boots' on our 'feet'.Some theories have it that the massive population movements into the South East of England after the Black Death may have led to the shift, with regional accents mixing with and influencing that region's standard pronunciation. There was also great social mobility in and around London following the plague, allowing previously marginalised pronunciations to influence the standard speech of a region that has since huge influence on accepted English. Others argue that the dropping of French by aristocrats in England may have influenced pronunciation. The toffs may either have tried to create their own posh accent more on the French model or, conversely, attempted to speak English in a way they felt was further from French (because France was an enemy).
The shift may be continuing. How do you pronounce "route"? Some Americans can rhyme it with "out". The English are still mired in a distinctly Anglo Saxon "boot". Anyway, all this change is one reason why English spelling is such a nightmare. It was standardised during this shift and therefore has little or no reliable connection to actual pronunciation. For more on that, see "ghoti" and English spelling reform.