A small number of Student sponsors are 'legacy sponsors'. Legacy sponsors have a zero confirmation of acceptance for studies (CAS) allocation and are not able to sponsor students to start a new course. If your institution has been made a legacy sponsor it’s important that you seek advice from a student adviser at your institution.
If your institution becomes a legacy sponsor and you currently hold, or have recently been granted a visa to study, you can continue to study and you can enter and re-enter the UK using your current visa.
If your sponsor issued you with a CAS to study a new course before becoming a legacy sponsor, the CAS will be invalid and cannot be used to make an application. If you decide to make an application with an invalid CAS the application will be refused. If you have already made an application to study a new course and are awaiting a decision, this will also be refused. Your sponsor will not be able to issue you with a new CAS in order for you to extend your Student permission, unless it is to re-sit an exam or repeat a module. This is outlined in the Home Office's Policy Guidance.
If you are about to complete a doctorate, you will not be able to apply under the doctorate extension scheme. This is because doctorate extension scheme applications can only be made in the UK, before your 'expected end date', and you must be sponsored by the same institution that sponsored your studies.
You should also check with your sponsor if their legacy sponsor licence is due to expire before the end of your course and whether they intend to apply to renew this. If they choose not to renew their licence this may mean that you are unable to complete your course.