Well, first, the definitions are somewhat arbitrary. There's no definitive line between phases.

But, yes. Generally speaking, English had a major event that caused a massive language change around 1400 called The Great Vowel Shift. This was a major change in how vowels were pronounced in the English language.

The thing about the languages is that there were massive regional differences that did not carry through. So, when speaking about Old English, you have to bear in mind that there were rather large variations in that. Old English in Mercia was different than in Wessex, which was different than in Northumbria. Even within those regions, there was a great deal of variations in terminology and pronunciation. Even in modern English, there are significant differences within England on how words are pronounced even within very short distances.

So, what happened was most likely the effects of the Black Death that starting in the mid 1300's. People left their home areas in larger numbers than normal and moved to areas with more economic potential, or areas perceived to be safer. So, there was suddenly an influx of people from all around England into various urban hubs. All of these people used their own native accents and terminologies when they first arrived in the urban centers, but these accents tended to coalesce into a new vowel sound for the words. Note that this also coincided with the widespread adoption of the printing press, so dictionaries started to be printed and fixing the word format and spelling the same time that how those words changed in pronunciation.