Brahma Sutras – According to Shankara 3-4-10

Topic 10 - One who has taken the vow of lifelong celibacy (Sannyāsa) cannot revert back to his former stages of life

 Sutra 3,4.40

तद्भूतस्य तु नातद्भावः, जैमिनेरपि, नियमातद्रूपाभावेभ्यः ॥ ४० ॥

tadbhūtasya tu nātadbhāvaḥ, jaiminerapi, niyamātadrūpābhāvebhyaḥ || 40 ||

tadbhūtasya—For one who has attained that (the highest Āśrama); tu—but; na—no; atadbhāvaḥ—ceasing from that; jaimineḥ—of Jaimini (is this opinion); api—also; niyama-atadrūpa-abhāvebhyaḥ—on account of restrictions prohibiting such reversion.

40. But for one who has risen to the highest Āśrama (i.e. Sannyāsa) there is no reverting (to the preceding ones), on account of restrictions prohibiting such reversion. Jaimini also (is of this opinion).

The question whether one who has embraced Sannyāsa can go back to the previous Āśrama is taken up for discussion.

This Sutra says that he cannot, because the Śruti expressly forbids it: “He is to go to the forest he is not to return from there.” But there are no rules allowing a reversion, like those which sanction the ascent to higher Āśramas. It is also against approved custom.

So one cannot revert from Sannyāsa.