When using Aegislash's "Stance Change" Ability, what do I do with the card after I switch it with an Aegislash from my hand?
The Aegislash that was in play goes into your hand.
Source: XY FAQ (2014-02-06), TPCi Rules Team (2014-02-06)

Relevant cards

Note: All applicable cards may not be listed below.
XY1_85
XY1_86