Just to be clear on my answer, "Jehovah" did not became "Yahweh" neither "Yahweh" became "Jehovah".
"Yahweh" is a popular Hebrew transliteration of the tetragrammaton in the old testament
while
"Jehovah" is a popular English transliteration in the new testament.
These are both the names of our Almighty God, just in different languages.