Gas Hookup at Water Heater

[Pittnb14]

I am replacing my water heater and need to replace the gas hookup also. Existing hookup pictured below. I'd like to use this tube: https://www.lowes.com/pd/BrassCraft...utlet-Stainless-Steel-Gas-Connector/999947696

Any idea why the small copper tube was used on my existing water heater? Okay to use yellow tube I linked, and leave existing ball valve in place? Any other advice on swapping out this hookup?

Thanks so much for your time and advice.