S. [ɸ], [β] became [f], [v]; [ɸ|β] > [f|v]
As suggested by David Salo (GS/§4.180), the phonetic developments of the primitive aspirate [pʰ] as well as the spirantal sounds that developed from [b] were most likely bilabial spirants [ɸ] and [β] to begin with, developing into labio-dental [f] and [v] later. The occasional appearance of ᵽ and ƀ for these sounds in Tolkien’s writing supports this idea, such as on PE22/28 and PE19/101. The timing of this change is hard to nail down; I put it early in my series of sound changes purely for convenience (“f” and “v” are easier to type than “ɸ” and “β”). Most likely this sound change occurred at all conceptual stages of Tolkien’s languages.