Costco Upgrade on a One Plan

Looking through, I see a lot of people talking about upgrading but without their plan.

Has anyone tried upgrading their phone on an older plan, and did you get any kind of perks like a gift card? The site is vague, and only talks about Go5G plans (and switching from another carrier)

Contemplating a Pixel 9 Pro/XL upgrade, and was hoping someone had some recent experience