1. A method of content substitution, comprising: receiving an ordered stream of packets containing content marked by a first packet identifier (PID) and one or more substitute content portions marked by one or more secondary PIDs, where the number and placement of packets marked by secondary PIDs ahead of packets marked by the first PID in the stream is retained during transmission of the ordered stream of packets; initiating processing for display or storage of content contained in packets having the first PID; initiating processing for display or storage of content contained in packets having a selected secondary PID that meets a substitution criterion; and either deleting or processing the content having the first PID depending upon a number of received intervening packets having secondary PIDs that reside between the packets having the first PID and the packets having the selected secondary PID that meets the substitution criterion.