What's the reason for changing f to v in plural nouns?

The reason F is changed to V in plural nouns is because of the way the sounds are formed in our mouth. The letters F and V are what's called 'labiodental fricative consonants' -- in other Words, letters whose sounds are created by forcing air through a constricted space (by using your lips and your teeth). When making the F sound at the end of a Word, we don't actually use our vocal cords; but when making the V sound, we do. All of the letters that are considered 'fricative consonants' (F/V, O/O, S/Z) are paired in this 'voiceless-voice' manner, and when pluralized all end in -ES.

Not all nouns that end in F are changed to V when written as plural. When a Word ends in FF, like STAFF, it can be pluralized in modern English by just adding an S.