Hey,
well this is not a usual dumb mmorpg question of dreaming childs,
we already got a mmorpg which is working pretty well its handling 65k players per instance
and we would like to move it from our own bad coded engine to Unreal Engine4
but the main problem i see in my eyes is the networking
should i use my existing server setup and modify my unreal game to it
or should i use the unreal approach and try to fit the server to it?
You need your own setup for 65k players per instance.
Just out of curiosity, do you really have 65k players or do you just assume your engine is able to deal with that amount in real time? 
the game is quite old (10 years) and yes it had indeed more then 200k players at the same time without any lags(not on the same server) (apart from problems with ddos attacks)
and what do you mean with my own setup ? do you mean a customized server and or a customized unreal engine? also how much customized?
Both actually, Unreal uses RPC on the same code base, So you program server and client code in the same file and then use switches to call a certain function if you are on the server or on a client machine.
Not exactly great for MMO’s.
But since you have the engine source code you could easily build a Messaged Oriented Middleware (or any other solution) to communicate with an existing server - you would just have to build a proper interface.
I suggest you not to use UNetDriver/UNetConnection, i.e. internal UE4 replication system, for MMORPG or any MMO.
UE4 networking is completely intended to be used for matchmaking, hosted by other players. (for example like Counter-Strike and other FPS games), for MMO its absolutely useless.
But UE4 sockets still pretty handy and crossplatform for creating your own networking for actors.
Also OnlineSubsystems can be useful for MMOs.
Thats what i have learned for now.