Soulshift is a leaves the battlefield trigger, so the gamestate is referenced immediately before the Soulshift trigger to determine the value of X. Soulshift X includes Kodama of the Center Tree. So, X is always at least 1.
2005-02-01 • wotc
Kodama of the Center Tree can return itself to its owner’s hand if you control five or more Spirits when it is put into a graveyard from the battlefield.
2005-02-01 • wotc