The command is /incarnate_equip slot power
You'll need to use underscores in the power names rather than spaces, if you're doing this in a macro or bind. For example:
/macro Pyro "incarnate_equip Judgement Pyronic_Judgement"
Here is an older thread on the subject.
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I have several incarnates now and some have several different powers available per slot, are there slash commands to change out the powers? I ask because slotting some of the different powers allown me to play my character in a completely different way.