I believe this KB answers your question: https://community.appdynamics.com/t5/Knowledge-Base/New-in-4-3-Universal-Agent/ta-p/23544 -- let me know if it doesn't.
In short - no it doesn't answer my question.
Specially, will the AppD 4.3 universal agent be capable of automatically upgrading Database agents?
Current version of UA (as of this writing) does not support managing DB Agents. Only Java Agent and Standalone Machine Agents are currently supported. Other agent support is coming, though I don't have any information on when DB Agents specifically may be supported by UA...