Overview
Whenever a player is converted from one side to the other, you do not have to begin again from scratch, but rather the player retains 50% of the SP earned from the side you were playing prior to being converted. If a human player gained 100sp, when converted into a zombie, the zombie would be granted 50sp to spend. If that zombie then gained an additional 400sp, when converted, the human self would gain 200sp to spend. Note that it isn't 50% of a players total XP that's transferred, but 50% of the XP earned during that one run on that side.