It would be a mistake to think the sphere of infinite consciousness is the same as the Brahman/Atman idea of the Upanishads, since the oldest records we have of these attainments (the Buddhist suttas/agamas) pair them up with the annihilationists. Check out DN 1. Part of the basis for views for the annihilationists are the formless attainments. They are not matched with anyone else. Further evidence is found in the suttas (im unsure about the agamas) which ties the annihilationists to them, for the most part. The eternalists seem to have preferred the Jhānā, and we can possibly detect hints of this in the Upanishads themselves:
यदा पञ्चावतिष्ठन्ते ज्ञानानि मनसा सह ।
बुद्धिश्च न विचेष्टते तामाहुः परमां गतिम् ॥ १० ॥
yadā pañcāvatiṣṭhante jñānāni manasā saha |
buddhiśca na viceṣṭate tāmāhuḥ paramāṃ gatim ||
When the five organs of perception become still, together with the mind,
and the intellect ceases to be active: that is called the highest state.