Its a design fault with the cable for the 4090, you can replace it yourself until NVidia acknowledge the problem.
If you have the option to choose between the 13900k or the 7950X the choice is obviously the 13900k. Performs better in almost all gaming workloads and a lot of productivity ones as well. Until they release the version of the 7950X with the memory sync stuff its basically pointless for gaming.
As long as your EKWB waterblock covers the socket of the 13900k (Socket 1700, 9900k and my EKWB is for socket 1151 so assuming yours is too) then you will be able to transfer everything over. You will need a new GPU waterblock and new liquid though.
Use this:
https://www.ekwb.com/configurator/ - Select your GPU, you MUST choose the correct one so do not buy one of these waterblocks until you actually buy a card. Because they won't always fit. Actually installing a waterblock on a GPU is easy though, just take off some screws, put some pads ontop of certain VRM etc. on the GPU's board and then screw down.
P.S: No fuckin idea what you plan to spend 7k euros on, I just put all parts together in a shopping cart on scan.co.uk (UK only) and it only cost 5k with an over £2000 4090
. If you were planning to buy brand new watercooling fittings etc. then yeah it would probably take you closer to 6k. Although I'm gonna be honest, watercooling is beyond pointless for a gamer.