大般涅槃經義記 1.119

1.119

自下第九，變土令淨。「三千大千」，淨之分齊。「以佛神力」，土淨所由。「地皆軟」等，正明土淨。「猶如西方」，類顯淨相。其土常淨。物本見穢，由前心善，佛力轉之，今始見淨，名為變土，理實變心。

From here down is the ninth: transforming the land to make it pure. "The trichiliocosm" is the scope of the purification. "Through the Buddha's supernatural power" is the cause of the land's purification. "The ground was all soft," and so forth, directly explains the purification of the land. "Just like the western direction" analogously illustrates the marks of purification. That land is eternally pure. Sentient beings originally saw it as defiled; due to their preceding good minds, the Buddha's power transformed it, and only now did they see it as pure. This is termed transforming the land, but in reality it was transforming their minds.