A Zotero database can only be synced with a single Zotero account.
Once you set up syncing for a given database in the Sync pane of the Zotero preferences, it's no longer possible to sync that database with another account. If you unlink your account and attempt to set up syncing with a different account, Zotero will warn you that all local data will be removed if you continue. This prevents data from different accounts from being inadvertently combined and avoids difficult-to-resolve conflicts between data created in multiple accounts.
If you're simply trying to change usernames, you can do so from your
profile settings rather than switching to a completely new account. (If you've already created a new account with the desired username, you'll need to delete it or change its username before you can use that username for your original account.)
If you wish to use libraries from multiple accounts on the same computer, you can set up
multiple Zotero profiles pointing to separate data directories and sync each one with a different account.
If you've created data in multiple accounts and want to merge the data into a single account, you can export the data in one account to Zotero RDF with files — either on another computer or from a separate profile that you later delete — and import it into the other account. Note that using export and import will reset Date Added/Modified times and will break links to items from citations in existing word processor documents, so you may wish to export from the account with less data or data that you've interacted with less.