In this regard (various names of the demigods, Brahma, Siva, Indra & so on are not personal names; they are names of different posts), we understand that Lord Visnu sometimes becomes Brahma or Indra when there is no suitable person to occupy these posts

Expressions researched:
"In this regard, we understand that Lord Viṣṇu sometimes becomes Brahmā or Indra when there is no suitable person to occupy these posts"

Srimad-Bhagavatam

SB Canto 8

The various names of the demigods—Lord Brahmā, Lord Śiva, Lord Indra and so on—are not personal names; they are names of different posts. In this regard, we understand that Lord Viṣṇu sometimes becomes Brahmā or Indra when there is no suitable person to occupy these posts.

SB 8.1.18, Translation and Purport

The Supreme Lord, Viṣṇu, who sits in everyone's heart, appearing as Yajñapati, observed that the Rākṣasas and demons were going to devour Svāyambhuva Manu. Thus the Lord, accompanied by His sons named the Yāmas and by all the other demigods, killed the demons and Rākṣasas. He then took the post of Indra and began to rule the heavenly kingdom.

The various names of the demigods—Lord Brahmā, Lord Śiva, Lord Indra and so on—are not personal names; they are names of different posts. In this regard, we understand that Lord Viṣṇu sometimes becomes Brahmā or Indra when there is no suitable person to occupy these posts.