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How did the pronunciation of beta change from Ancient Greek to Modern Greek?

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Thematic map of Eurasia showing the approximate geographic distribution of Indo-European language families, each represented by a different color. Areas include Celtic in western Europe, Germanic in northern and central Europe, Romance in southwestern Europe, Slavic in eastern Europe and Russia, and Indo-Iranian languages in South Asia. Non-Indo-European language regions and areas of common bilingualism are also indicated. — Greek language Approximate locations of Indo-European languages in contemporary Eurasia.

In Ancient Greek, the letter beta ( $β$ ) represented a voiced bilabial stop, the same sound associated with the letter "b" in English.1

Over time, the pronunciation of this letter shifted significantly. By the time of the Byzantine phase of the Greek language, the phonetic value of beta had evolved into a voiced labiodental fricative, or the "v" sound. Consequently, in Modern Greek, the letter is transliterated as "v" rather than "b," reflecting its current pronunciation. This historical shift is also associated with the broader linguistic phenomenon known as betacism, which describes the loss of distinction between "b" and "v" sounds in various languages.123