A method and system for generation of a block mode conversion table for use in online transcoding of a sequence of input images in a first format to a sequence of output images in a second format are described. The table is based on a sequence of training images and provides an effective mapping between the encoding block modes and respective candidate transcoding block modes used during the online transcoding. The method utilizes the encoding block mode, the motion vectors and the residual information extracted during the decoding of the training image to generate the block mode conversion table. A corresponding system is also provided.