Yamaha and other resellers do not list the models you mentioned as 8k capable anywhere. The few 8k TV and AV boxes (so far only denon AV box has 8k of any kind) is not fully supported or implemented.
Broadcasters in North America don't utilize 4k much.
Misc: AV box makers that include a "future proofing" type feature don't have a good track record of making good on those promises of compatibility or updating it VIA software if such an option exists to support new stuff. It is far more profitable to just sell a new box then support the units already sold.