In fact I have got the same kind of questions. I have always wondered why the translators of the Hebrew bible (and the New Testament authors following them) changed so many names. Why Rebekka from Rivka(h)? Why Elisabet, Elisabeth (different T-character in the end! Why?) and Eleisabeth for Elisheva? - There must be some literature about this, but I haven't dug up anything.
Some characters of the Hebrew language just don't exist in Greek or Latin, so this explains some of the changes. Also I can imagine, that in many cases the translators would adapt names to something like "Greek style", but this doesn't explain it all. The Greeks obviously loved the letter S at the end of male names and made up THOMAS from THEOM and JONAS from JONA. Maybe some were just a little careless, a steady source of inaccuracies.
With S or Z in Elizabeth I once read something, why it was changed in the 16. (?) century. It had something to do with QueenElizabeth, but I don't remember. Anybody else knows?
Sometimes names are changed because of wrong etymology (even deliberately): Take ANTHONY which had no H until the 15. cent., when someone thought, it had something to do with the Greek wort "anthos" (flower). Or CARMEN, which was (Lady of the) Carmel, but was assimilated to the word for "song, charm".
I only recently learned about JOHN and JACK, and I found this quite interesting.