When you plan to transmigrate, do it at a higher level than the level you previously transmigrated from otherwise your stats will decrease. For example, if you have a Prinny God at level 3,000, transmigrate and level him to level 1,000; if you were to transmigrate him at level 1,000 his stats would decrease rather than increase. Also, it is highly recommended you always transmigrate under "Genius".