Justin, while KEAM website isnt clear about all the corrections that you can make in your application form. But Tahsildar stays on the top of  the hierarchy. If you have a certificate issued by Tahsildar, it still stays valid because if Village officer is the head of a village, a Tahsildar is the head of a block of villages. So that should be absolutely fine. But if you still want to upload a certificate issued by the Tahsildar, usually uploaded images can be changed only if notified. Some uploaded images also have "Edit" option which means you can change the uploaded images during the correction window. KEAM will open its correction window during the 1st week of March tentatively. So please watch out for official information.