Substitute in is a phrase used to refer a situation when a new attorney or a party is introduced in an existing suit. When an attorney is excused from a trial and a new attorney is selected to replace him/her to continue the trial in that court, the party must file a written statement to that effect.
When the facts of the cause of action against the defendant would not be wholly different after amendment, a complaint filed by a party without standing may be amended to substitute in the real party in interest. [Cloud v. Northrop Grumman Corp., 67 Cal. App. 4th 995 (Cal. App. 2d Dist. 1998)].