Yes, you can appear for the CBSE Class 12 exams as a private candidate, even after giving your NIOS On-Demand exam, and switch to the Commerce stream. CBSE allows students to switch streams when registering as private candidates, as long as they meet the eligibility criteria.

Here’s what you need to consider:

1. Eligibility: To appear as a private candidate, you must have completed the previous academic year and be eligible to take the exam. Since you have already completed your 12th through NIOS, you can register with CBSE as a private candidate.

2. Subject Choices: You can choose subjects from the Commerce stream, but you will need to ensure you meet any prerequisites for the subjects you choose. For example, subjects like Mathematics might require prior knowledge.

3. Registration: You’ll need to register as a private candidate through the official CBSE portal during the registration window. Keep an eye on the CBSE website for notifications and guidelines specific to private candidates.

4. Documentation: Ensure you have all the necessary documents, such as your previous academic records and proof of passing 12th through NIOS, when applying.

Switching streams is possible, but make sure to carefully choose subjects that align with your career goals and abilities.