Once gurupuj was only allowed on the appearance day of Srila Vyasadev. S. Puri Maharaj personally told me that pranama mantra was introduced by S. Bhaktisuddhanta learned disciple who wrote pranama mantra for his guru and then they had to write it for respect to gran guru, gran gran guru etc, but before that vaisnava would only use vancha kalpa tarubiasa mantra to greet each other. Once they introduce this it became part of tradition but it us quite recent and there was not such thing at the time of sat Goswami of Vrindavan.
If there is no specific order or guru after his demise a qualified disciple may become a diksha but at at advanced age (he told me 70 years old) by popular acclaim for he is being recognized for his self effulgence, his high character and his mastering the scriptures
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