Hello Shushan,
Here are the answers to your questions to help you get this sorted out before your event:
1. Will the webinar with the new Microsoft Premium account accommodate 1000 attendees?
It depends on exactly which "Premium" license you added to your account:
If you upgraded to Microsoft 365 Business Premium, the limit unfortunately remains at 300 interactive attendees. Both the Business Standard and Business Premium base plans cap webinar attendance at 300.
If you purchased the Microsoft Teams Premium add-on license and assigned it to the organizer's account, then yes, it will accommodate up to 1,000 interactive attendees (and allows thousands more to join in a "view-only" experience once that cap is hit).
(Note: If you do not have the Teams Premium add-on, the other way to achieve 1,000 interactive attendees is by upgrading the organizer's base license to an Enterprise plan, such as Office 365 E3/E5 or Microsoft 365 E3/E5).
2. Does the webinar link need to be changed?
No, you do not need to change the link. The meeting link and registration URL are tied to the unique meeting ID, not the specific license you held at the moment of creation. As long as the upgraded license or add-on is actively assigned to the organizer's account, the backend capacity will automatically increase for your existing webinar.
Because your event is in 2 days, I highly recommend verifying in your Microsoft 365 Admin Center that the correct license is actively assigned to your specific user account so you don't run into any capacity roadblocks.
Hope your webinar goes smoothly!