Philippians 2:5-9 describes the attitude of humility, love, and service which Christ Jesus possessed while on earth:
Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name that is above every name...
There is much already said about what it means to "be in the form of God" and "equality with God" in the literature of both trinitarians and non-trinitarians. See here for a start. This question is not about those particular aspects of what some theologians, in expounding this passage, have termed "the Kenosis".
The question is regarding the notions of "emptying" and "taking the form of". The passage seems to present that in order for Jesus to be found in the form of a servant he needed to empty himself. More specifically, it states that he emptied himself by taking the form of a servant and being born in the likeness of a man.
If Jesus did not pre-exist his birth then:
By taking the form of a servant and being born in the likeness of a man he emptied himself of what, exactly?
How can one empty oneself by birth as a man if one does not pre-exist that birth?