I mean the warranty. But the TV is too.
It's the minimum I'd upgrade too coming from the H6400. Although you may want to consider a higher end TV to get the biggest benefit from an upgrade as the RU8000 is still just a mid range model lacking good HDR hardware.
Buying a TV isn't just about resolution. A 4k model isn't going to be tremendously better than a HD one, especially one with similar kind of specs and only 55" in size. Despite resolutions being larger on TVs now very little has improved since 2014 with LCD TVs.