Merger of /b/ and /v/
Most Romance languages (but not Spanish) have maintained the distinction between a phoneme /b/ and a phoneme /v/ — a voiced bilabial stop and a voiced, usually labiodental fricative, respectively. Instances of the /b/ phoneme could be inherited directly from Latin /b/, or they could be the result of a voicing change on the Latin /p/ (spelled B and P respectively). The /v/ phoneme was generally derived from the Latin phoneme corresponding to the letter V and thought to have been pronounced [w] in Classical Latin, but later "fortified" to fricative consonant status. In those languages where this phoneme came to have labiodental articulation, it is suggested that that quality may have resulted from influence by the voiceless labiodental /f/. It is further suggested that influence from the Basque language may have prevented the labiodentalization of the voiced phoneme in Spanish, resulting in a bilabial fricative [β] that was indistinguishable from the spirantized instances of the /b/ phoneme (Lloyd, p. 239). In Modern Spanish the letters b and v represent the same phoneme, /b/ (whose realization is generally spirantized, except when utterance-initial or after a nasal consonant), with the choice of orthographic b or v depending mainly on the etymology of the word.
which sounds like your normal consonant shift. Anybody who knows the story?