Indeed. There is also an advance package devoted to the development of a form of possession left by incidental contact with the daemonic (such as the self-exorcism performed as part of the Shadow Over Thy Soul background package). So if you need a way to work things in, you might have it transpire that the initial exorcism left a growing daemonic seed which needs to be removed properly by a more complete ceremony of exorcism. Or you can just apply the effects retroactively, if this suits your character's history better.