Where does it say that you have 480 installed - after you installed AMD-supplied rather HP-suppied driver perhaps?
As far as my limited google search shows it's the same card / hardware, but 580 can work faster. If that's so and unless you install non-HP-supplied driver, you will suddenly see 480 there.
To restore it back, either roll the driver back, or pick the appropriate one from here:
https://support.hp.com/gb-en/drivers/selfservice/omen-by-hp-870-200/13687063/model/15193882
I'm guessing this one:
https://support.hp.com/gb-en/drivers/selfservice/swdetails/omen-by-hp-870-200/13687063/model/1519388...
I encountered had the same pickle with my R9 390 that suddenly shwed up as R200 once I installed AMD driver. Rolling it back to HP version resolved it.