As far as forcing clients to look to a new ePO server check out this link:
I have used that method to force 5.x Agents to talk to a secondary ePO server I had setup, once they talk it would pull whatever policies that ePO server has configured
As far as moving ePO itself, I'm not sure how that would work. I think you would need to do a fresh install and point it to the same database while you took the main ePO offline and upgraded that to 5.3
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first of all. You cannot use a EPO DB for two EPO Server installations. You have several Options how this can be done.
From my side of experience the easiest ways would be to install a new EPO Server. Afterwards Register each EPO Server to the other. So you are able to share the policies and you can migrate the agents to the new EPO Server without any deployment on the endpoint.
The only disadvantage is, there are no threat Events available any more in the new database.
If you want to "Transfer" the whole database including all Events as well you may do a complete disaster recovery as explained in the McAfee Guides.