I would pronounce
Joanna and
Johanna differently.
Johanna I usually pronounce as "jo-HAN-uh," but I can see the more authentic
German pronunciation, "yo-HAH-nuh" as well.
I pronounce
Alice and
Alyce the same way.
And there's the rub. I don't have a general rule to answer your question with. It just depends on the name and the particular spelling presented.
As a side note, can I ask anyone reading this to answer me one question. I know French
Jeanne is pronounced, "zhann." But there's a pretty looking medieval form of the name spelled Jehanne. Seomtimes I've seen
Joan of Arc's name spelled this way. I've never known how to pronounce it though. I've been using the same "zhann" one for Jehanne, is that correct?