Hi
First of all, I'd like to extend you a warm welcome to the Community Forum. I hope you find it a valuable resource.
A few reasons could prevent you from changing to the new serial number and upload the file online, it could be that the new serial number is for a different product level than your company file (Basic/Standard/Plus/Premier), maybe there isn't an active subscription against the new serial number, or as Harshana mentioned, you are not set up as an online file owner against your serial number.
In order for us to investigate further, could you please private message me your serial number, and the email address you use to upload file online? You can send a PM by clicking on my name > Send this user a private message.