1. A method of content substitution, comprising: receiving data representing video content, the data having a plurality of packet identifiers (PIDs) where a first PID is associated with a stream of original content, where the original content is made up of frames of content with each frame being made up of a plurality of original macroblocks; where a second PID is associated with substitute content, and where the substitute content comprises one or more substitute macroblocks of content but constitutes less than an entire frame of macroblocks; initiating processing of the original content having the first PID; determining that a substitution criterion has been met to replace one or more macroblocks in the at least one frame of original content with one or more macroblocks of substitute content, wherein the replacing modifies at least one frame of the original content by replacement of less than all original macroblocks in a full frame of the original content; replacing the one or more macroblocks of the original content having the first PID with one or more macroblocks of substitute content having the second PID to produce modified content; where, the replacing is carried out by selectively switching a controller from a bypass mode to a mode that effects replacement of the substitute macroblocks for the original macroblocks; and processing the modified content.