If your course of study or your focus of research has changed once you arrive in the UK then you should consult with the Student Immigration Team as there may be implications for your visa. You may need to apply for a new ATAS certificate.
If you have to extend your Tier 4 visa to complete a course of study which is subject to ATAS clearance then you’ll have to get a new ATAS certificate. You should complete a Confirmation of Acceptance for Studies or CAS request form which should be submitted to the Student Immigration Team in Academic Registry. You’ll not be able to apply for your new visa until you have received your ATAS clearance certificate.