This is possible without redoing any motion estimation for that frame. In fact, the new frame could use references to future frame, but does not need to use these. One P frame now is written as a B frame by an attacker. References to the next P frame are allowed but not used in this case. Neighbouring frames will then have incorrect references.
Figure gives an example of such an incorrectly decoded frame.
Erroneous references create blocks of 8 by 8 pixels with substantial luminance and chrominance errors, particularly in areas with motion. One P frame now is written as a B frame. Frames marked with grey circles have incorrect references and will show severe artefacts. Despite the attack, the detector perfectly recovers the watermark message. To correct the artefacts, the processing effort needed is comparable to MPEG encoding from scratch.