Basically, what I am asking is whether all humans have lived as other beings or not. I heard that everyone attains human birth after progressing through 84 lakh (or 8.4 million) stages of being born as other beings (animals, plants, etc.). I may be incorrect.
Yes it is true.
Garud Purana 2:49:13
चतुरशीतिलक्षेषु शरीरेषु शरीरिणम्॥
न मानुष॑ विनान्यत्र तत्त्वज्ञानन्तु लभयते॥ १३॥
In the eightyfour lacs of bodies of creatures one does not acquire true knowledge anywhere unless one is born as man.
Not only humans, even gods have to take birth in different yonis to attain human birth. Hence human birth is considered superior to other yonis including deva yoni.
S.B. 11:10:26. Residents of heaven enjoy the celestial delights until their merits have been exhausted. Then they are reluctantly forced to fall back to the lower abodes by the passage of time.”
B.G.9:21. When they have enjoyed the vast pleasures of heaven, the stock of their merits being exhausted, they return to the earthly plane. Thus, those who follow the Vedic rituals, desiring objects of enjoyment, repeatedly come and go in this world.
Shiva Purana:Uma Samhita:Chapter 20. It is difficult for even the gods and the demons to achieve the human body. Therefore after achieving the human body, one should act in a way that he has not to face the agonies of the hell.