You can change it per domain so that you can control what each domain uses. But it will not exceed whatever the main account is. You are getting domain usage confused with account usage.
NO an end user CANNOT upgrade his disk limit. The account for the user is still limited even if he tries to upgrade the disk limit per domain.
Suppose you set the limit for the end user to 20 GB of disk space and then the end user decides to increase the disk limit on his DOMAIN to 30 GB. He is still LIMITED to 20 GB be cause the ACCOUNT is limited.