this is my own understanding of the transition. hopefully someone else will come by with a more scholarly answer, but until they do...
John is Youkhanan in my language. Youkhanan -> Youhanan -> Youhan or
Johann (J pron as Y) -> Iovan (like
Giovanni) or
Jovan ->
Evan.
also...Johann ->
Jan (as Y) ->
John.
that is their relation to each other. :)