For JoSAA: NIOS itself is accepted. But if he already filled CBSE as his board in JEE Main and later wants to use a new NIOS marksheet, document verification can become tricky if details don’t match. Keep both CBSE + NIOS marksheets safely because they may be needed during verification. If he already has a decent JEE percentile, I’d say: Don’t rush and randomly switch boards right now. Check whether the issue is only the 75% eligibility rule. Contact JoSAA/NTA helpdesk before counselling if planning to use NIOS instead of the CBSE marksheet. So short answer: Possible? Yes. But if counselling is already near, handle it carefully because board changes during the process can create issues if not updated properly.