I'm not exactly sure how old is you system, or for the same matter what kind of CPU does it have.
Old technology in many places of our big wide world may have different considerations. Where I live my current PC is considered current, new and powerfull tech, cause most people can not even afford a +3 year old Core i3 10100.
So your question may have a few posible answers. The only thing I can do is make you a few question:
1. Does the old system support the RX 6800 XT? If the answer is not, then you need a new system.
2. Will your current CPU + Display resolution and game settings going to limit the RX 6800XT? Then you need a new CPU+Mobo and probably RAM.
3. Will you take advantage of the Ryzen 7 7700X, other than gaming?
4. Are the 13600k or the Ryzen 5 7600X (or the likes) cheapper than a R7 7700X where you live?
The other thing I can do is give you my example. I own a R5 3600 and 5600G, I use and tried both cpus with a RTX 2070 on a 1440p display + Ultra/High game settings (I don't really care about FPS, I want my games to look the best they can, as long as I get around 60 FPS). If I were to get a free RX 6800XT and because I am already getting 60 FPS in all the games I like and enjoy playing I would not even think about changing cpu+mobo+ram. Thats my particular scene, I do not know yours.