Just so you know, everyone was wrong including myself. Here is the real answer. Shoutout to Overclockers.net and the EKWB forum over there with real EKWB support reps.
http://www.overclock.net/t/1624359/am4-upgrade-kit
TL;DR
I see that you have a Supremacy MX, if it's the case, you don't need the gasket and you don't need to switch the backplate. You can keep the original one.
All you need to do is to install the new mounting plate like you did.
When you'll have your motherboard, you'll have to follow the instructions (https://www.ekwb.com/shop/EK-IM/EK-IM-3831109800317.pdf) to install the block on your board.
Basically, you will need to remove the original plastic hold-down clamps around the socket and install the CPU block.
HOWEVER; KEEP READING!!!!
Can i ask why one needs the gasket and why one does not. I do have the MX.
Answer:
Simply because the mounting mechanism isn't the same on the MX and the EVO. The MX has the screws already mounted and ready to be screwed on the original backplate while the EVO requires to install screw-like posts that screws in the EK backplate. The you slide your CPU block over the 4 posts and you secure everything with the provided springs and thumb screws
IF YOU HAVE THE EVO YOU NEED THE RUBBER GASKET IF YOU DON'T HAVE THE EVO YOU DON'T! AGAIN IF YOU HAVE THE EKWB SUPREMACY MX YOU DO NOT NEED THE BACKPLATE OR THE RUBBER GASKET. IF YOU HAVE THE EKWB SURPREMACY EVO YOU NEED THE BACKPLATE AND THE RUBBER GASKET!
Don't fry your crap kids. Someone should sticky this.