Yes, you can choose a different optional subject for the mains exam even if you have already chosen Zoology for the preliminary exam.

Choosing an Optional Subject for the UPSC Mains Exam
The UPSC (Union Public Service Commission) allows candidates to choose one optional subject for the mains exam. This subject is chosen from a list of subjects provided by the UPSC. The optional subject carries a significant weightage in the mains exam and is crucial in determining the final rank of the candidate.

Changing the Optional Subject for Mains Exam
If you have already chosen Zoology as your optional subject for the preliminary exam, you can still choose a different optional subject for the mains exam. The UPSC allows candidates to change their optional subject for the mains exam, provided they meet certain criteria and follow the necessary procedure.

Criteria for Changing Optional Subject
To change your optional subject for the mains exam, you need to fulfill the following criteria:

1. You must have chosen the same optional subject for the preliminary exam as well as the mains exam.
2. You must have obtained the minimum qualifying marks (as decided by the UPSC) in the preliminary exam.
3. You must have a valid reason for changing your optional subject.

Procedure for Changing Optional Subject
To change your optional subject for the mains exam, you need to follow the below procedure:

1. Fill the DAF (Detailed Application Form): After clearing the preliminary exam, you will be required to fill the DAF. In the DAF, you need to mention the optional subject you have chosen for the mains exam.
2. Submit the DAF within the given timeline: You must ensure that you submit the DAF within the specified deadline. Late submissions may not be considered.
3. Wait for the approval: The UPSC will review your request for changing the optional subject and make a decision based on the validity of your reason and the availability of the desired subject.
4. Confirmation of the change: Once the UPSC approves your request, you will receive a confirmation regarding the change of your optional subject for the mains exam.

Conclusion
In conclusion, if you have already chosen Zoology as your optional subject for the preliminary exam, you have the option to choose a different subject for the mains exam. However, you must fulfill the necessary criteria and follow the designated procedure to change your optional subject.