Even though you can create a PID and password as an incoming student, access to the library's restricted resources like databases and ejournals will not be available until the Registrar's Office runs an end of term/start of term job that changes incoming students' VT affiliation. That can depend on when your assistantship starts (usually by the first day of classes) or when you pay your tuition (can be after classes start).
You can check your current affiliation by logging into OneCampus, search for the Manage Accounts category, and then click on Profile. Scroll to the bottom to see a list of your current affiliations. You'll need to see VT-ACTIVE-MEMBER, VT-AFFILIATE, or VT-RETIREE listed under Affiliations to be able to have Off Campus Access to Library Resources.
The library has absolutely no ability to change your affiliation. Contact your academic department for assistance. Students should speak with the Registrar or Graduate School.
An explanation of university affiliations is available online.