Original: Shortly before his departure from the US in January 1918, he re-wrote and renamed Urbs Dei, significantly altering the first verse to concentrate on the huge losses suffered by British soldiers during the intervening years.
Simplified: Shortly before he left the United States, in January of 1918, he re-wrote and re-titled Urbs Dei, after making big changes to the first verse to talk more about the huge losses that British soldiers were having in the war years.