You choose the ability's mode when you activate it.
2011-09-22
wotc
You can activate Skinshifter's ability during any player's turn, but only once on each turn.
2011-09-22
wotc
Skinshifter's ability doesn't cause it to lose any other abilities it may have.
2011-09-22
wotc
Skinshifter's ability doesn't affect its copiable values. Creatures that enter as a copy of Skinshifter will enter as 1/1 Human Shaman creatures.
2011-09-22
wotc
No matter which mode you choose, Skinshifter will retain any other card types it may have. For example, if an effect causes Skinshifter to be an artifact in addition to its other types and you activate its ability choosing the first mode, it will be both an artifact and a Rhino creature.
2011-09-22
wotc
No matter which mode you choose, Skinshifter will cease to have any creature types it previously had. If you activate its ability choosing the first mode, it will become only a Rhino; it's no longer a Human or a Shaman.