You're trying to change the object to which a placed object is linked? So if I'm understanding you right, you'd like to take a place image, say, that's linked to file A and change it so it's linked to file B instead?
If I've got that right, then AIPlacedSuite::ExecPlaceRequest() might be what you want. I've never used it, but it looks like it uses a class (AIPlaceRequestData) that includes 'request type' enum that includes:
kQueryReplace
kForceReplace
kUnlinkPlaced
kForceReplaceEx
and some others. Might that be what you want? It also seems to have members for 'old art' and 'new art'. Sort of suggests you can use it to alter a placed object.