I have been wondering. It is said in many tantric texts that if we transgress our Guru's instructions, there will be no effect from our Dharma practice and neither will there be any effect on people's minds when we talk to them about the Dharma. If a 'higher' Guru cannot override the instructions of the 'lower' Guru as explained by the Lamrim, if Lama Zopa gives up Dorje Shugden then it would mean that whatever FPMT does will not benefit anyone and Lama Zopa's pujas and practices will not bring in any positive effect for anyone and his talks will not benefit others as it has no power. The fact that he still can means that he has kept his samaya intact, and therefore, it is not possible that he has gave up.