First: the permissibility of this was exclusive to the Companions. This is how those who prohibited converting Hajj into ‘umrah understood it.
Second: its obligation was exclusive to the Companions. This was the view of our teacher, may Allah sanctify his soul. He said (1): They were obliged (2) to convert their Hajj into ‘umrah because the Messenger of Allah, peace and blessings be upon him, commanded them to do so, made it binding upon them, and became angry when they hesitated to comply immediately. As for its permissibility and recommendation, these apply to the Muslim community until the Day of Resurrection. But Ibn ‘Abbās, that ocean of knowledge, rejected this view and held that the obligation applied to the Muslim community until the Day of Resurrection: everyone performing Hajj alone or combining Hajj with ‘umrah who has not brought sacrificial animals must leave ihrām (ritual consecration). Indeed, he has already left it, even if he does not wish to. I am more inclined to his view than to that of our teacher.
The third possibility: no one after the Companions may begin Hajj alone, or Hajj combined with ‘umrah, without sacrificial animals, such that he would need to convert it into ‘umrah. Rather, he must follow the Prophet’s final instructions to his Companions, peace and blessings be upon him: tamattu‘ for those who have not brought sacrificial animals, and combining Hajj with ‘umrah for those who have, as is authentically reported from him. As for entering ihrām for Hajj alone, then converting it at the time of circumambulation into a separate ‘umrah, thereby making it tamattu‘ = this is not permitted for him. That was exclusive to the Companions, for they had entered ihrām for Hajj alone before the Prophet, peace and blessings be upon him, commanded them to perform tamattu‘ and convert their Hajj accordingly. Once his command to perform tamattu‘ and convert their Hajj accordingly was firmly established, no one could contravene it by entering ihrām for Hajj alone and then converting it.
If you consider these latter two possibilities, you will find them either more compelling than