1. A computerized method for transcoding a sequence of input images, each of the input images comprising input macroblocks of pixels encoded in a first format, into a sequence of output images, each of the output images comprising output macroblocks of pixels encoded in a second format, the method comprising: transcoding in a computer having a processor, comprising: reading a set of transcoding use cases, each transcoding use case is characterized by a respective type of adaptation to be performed on the input image; decoding input macroblocks, comprising extracting metadata with encoding block modes and determining partitions and respective motion vectors, thereby decoding the input image into a decoded input image; based on the set of transcoding use cases, determining a number of stages for mapping motion vectors; performing a block of operations on the decoded input image using the extracted metadata, the block of operations corresponding to the set of transcoding use cases and comprising mapping motion vectors and macroblock modes to generate an adapted input image, each operation of the block of operations corresponding to a transcoding use case in the set of transcoding use cases; and re-encoding the adapted input image to generate the output image.