Change of position is a defence to a claim in unjust enrichment, or for restitution. Ordinarily, someone who has been unjustly enriched at the expense of another is strictly liable to disgorge his gains. However, a "change of position" defence operates where it would be inequitable to compel him to make restitution.
In English law, explicit recognition of the defence in response to an unjust enrichment claim was only first made in Lipkin Gorman v Karpnale Ltd [1991] 2 AC 548.