"Before I formed you in the womb I knew you..."
could more readily be an testimony of God's existence outside of time rather than prior existence of the soul.
The existence of a plan is also no indication of a souls preexistence or an indication of when it is present in a persons' body.
For the record, I believe it is a person at conception and that deliberate abortion is murder.
It is a person at conception....but each soul is a part of God's plan....past, present, future.