To edit the preferred credentials you should be able to go to (assuming you have permissions):
Setup -> Security -> Preferred Credentials
To override, you should be able to log out of the entity (not OEM), and then log in manually using sys (with role as sysdba).
Thanks for your advice, however my cloud control does not appear to offer the functionality of logging out of the database - it's permanently logged in as SYSTEM. I can log out of SYSMAN but that's not so useful....
when I'm on the "Advisors Home" page of one our databases in OEM (database homepage -> performance -> Advisors Home) and I click on the OEM username (top right) a drop down list with the item "logout" appears.
I can then choose if I
- want to log out the database
- log out of OEM
In the first case, OEM can display a database logon page which allows me to connect to the database as SYS.
I'm pretty sure that your OEM Cloud Control works in the same way.
Sir you are a legend! Seems straightforward now but honestly it's just not intuitive to work out.