Vishnupurana 3.8.40

"In times of distress the peculiar functions of the castes may be modified, as you shall hear. A Brahman may follow the occupations of a Kshatriya or a Vaiśya; the Kshatriya may adopt those of the Vaiśya; and the Vaiśya those of the Kshatriya: but these two last should never descend to the functions of the Śúdra, if it be possible to avoid them; and if that be not possible, they must at least shun the functions of the mined castes. I will now, Rájá, relate to you the duties of the several Ásramas or conditions of life."

"In times of distress the peculiar functions of the castes may be modified, as you shall hear. A Brahman may follow the occupations of a Kshatriya or a Vaiśya; the Kshatriya may adopt those of the Vaiśya; and the Vaiśya those of the Kshatriya: but these two last should never descend to the functions of the Śúdra, if it be possible to avoid them; and if that be not possible, they must at least shun the functions of the mined castes. I will now, Rájá, relate to you the duties of the several Ásramas or conditions of life."