It has been clearly asserted by Biblical Unitarians that Jesus Christ had no actual existence anywhere prior to his birth in Bethlehem. It has also been clearly asserted that Scriptures which appear to present some kind of pre-existence for Jesus (John's prologue, for instance) are really referring to his "notional existence" within the mind of God.
There are many questions and answers which flesh this out. Here are a few: how-do-those-who-believe-that-jesus-is-a-created-being-understand-the-verses-whi, why-don-t-unitarians-believe-that-jesus-christ-pre-existed-before-incarnation, according-to-biblical-unitarians-how-much-does-notionalism-encompass-in-john
The general idea is that God has always had the plan to cause Jesus' birth and this plan is what is referred to as the Word in John's Gospel. The Word also must incorporate the "plan in God's mind" for literally everything since all things were brought into existence through this same Word. For instance, God had the "plan" for light and when He said "Let light be" that plan was actualized.
When we are told that the Word became flesh it is here that the "plan" of God regarding Jesus was actualized and where Jesus went from "notional" to "actual" existence.
This Biblical Unitarian article presents a Word which encompasses all of God's plans and, indeed, God's rational thought itself. My question therefore is: According to Biblical Unitarians, when the Word became flesh (when the plan was actualized) was it all of the Word or part of the Word which became flesh?