This small snippet is from the start of Sri Gur Nanak Prakash Granth, describing how there was no difference between Sri Guru Nanak Dev Ji and Sri Guru Angad Dev Ji, only the physical form had changed:
ਚੌਪਈ ॥ ਵਿਚ ਕਰਤਾਰਪੁਰੇ ਜਗਰਾਈ ॥ In Kartarpur, the king of the world, Sri Guru Nanak Dev Ji
ਸ਼੍ਰੀ ਅੰਗਦ ਕੋ ਦੀਨਿ ਬਡਾਈ ॥ gave the glorious gift (of the Gurgadhi) to Sri Guru Angad Dev Ji.
ਅਪਨ ਕੀਨ ਸਚਖੰਡ ਪਯਾਨਾ ॥ Sri Guru Nanak Ji then went to Sachkhand
ਜੋਤਿ ਰੂਪ ਬਿਬ ਤਨ ਪਲਟਾਨਾ ॥੬॥ infusing Their light into Their second body (Sri Guru Angad Dev Ji).
ਜਿਉ ਅਧਿਪਤਿ ਇਕ ਪੋਸ਼ਿਸ਼ ਤਜਿਕੈ ॥ ਬੈਸਹਿ ਸਭਾ ਦੁਤਿਯ ਤਨ ਸਜਿਕੈ ॥ Just like a King, taking off one of his royal robes and wearing a second robe, he sits in his Court.
ਤਿਉ ਖਡੂਰ ਮਹਿਂ ਆਨ ਬਿਰਾਜੇ ॥ In the same way, the second form of Sri Guru Nanak Dev Ji came to reside at Khadoor Sahib,
ਜਿਨ ਸਿਮਰਨਿ ਕਲਿ ਕਲਮਲ ਭਾਜੇ ॥੭॥ that Guru Ji whose Simran (meditation) sends the sins of Kaljug (the dark age) running far away.
ਦੋਹਰਾ ॥ ਤਖਤ ਬੈਠਿ ਅੰਗਦ ਗੁਰੂ ਕੀਨ ਖਡੂਰ ਨਿਵਾਸ ॥ Sri Guru Angad Dev Ji sitting on Their throne came to live at Khadoor Sahib,
ਨਰਨ ਉਧਾਰਨ ਕਾਰਨੇ ਕਰਤਿ ਸੁ ਭਗਤਿ ਪ੍ਰਕਾਸ ॥੮॥ for the sake of saving the people, Guru Ji manifested the path of Prema Bhagti (loving devotional worship).
Bhai Gurdas Ji in Vaar 1 explains beautifully, how Sri Guru Nanak Dev Ji transformed into Sri Guru Angad Dev Ji:
ਥਾਪਿਆ ਲਹਿਣਾ ਜੀਂਵਦੇ ਗੁਰਿਆਈ ਸਿਰਿ ਛਤ੍ਰੁ ਫਿਰਾਇਆ । Sri Guru Nanak Dev Ji during Their lifetime waved the royal canopy above Lehna (Sri Guru Angad) establishing Them as Guru.
ਜੋਤੀ ਜੋਤਿ ਮਿਲਾਇ ਕੈ ਸਤਿਗੁਰ ਨਾਨਕਿ ਰੂਪੁ ਵਟਾਇਆ । Merging Their light into Lehna’s Sri Guru Nanak Dev Ji transformed Themselves.
ਲਖਿ ਨ ਕੋਈ ਸਕਈ ਆਚਰਜੇ ਆਚਰਜੁ ਦਿਖਾਇਆ । No one could comprehend this mystery. Sri Guru Nanak, the miraculous, performed a miracle.
ਕਾਇਆ ਪਲਟਿ ਸਰੂਪੁ ਬਣਾਇਆ ।੪੫। Sri Guru Nanak Dev Ji has converted (their body) into a new form.
This snippet has been translated by Bhai Gurdas Educational Trust, from their first volume of Sri Nanak Prakash English translation. Please follow their account for more posts on Sikh history.