tesla
it wouldn't be a "substitution" it would be a definition.
But the term God has historically never been applied to mere existence but to the mystery "behind" existence. As such it has been subjugated to being a condition separate from existence. By stating that God is
in effect defined as existence you are narrowing the focus down to phenomena and deciding that the supernatural aspect{ God's separation from existence} is not applicable.
I am not sure that this will find acceptance in religious circles since it circumvents the established doctrine, and it is hardly science to assign a new name to existence by equating the term God
with the same meaning since it adds no knowledge to our understanding.