Want to upgrade phones and have an up-to-date phone plan. Costco or ATT?

I have a Costco membership and I see the ATT kiosks there all the time. I know Costco has an upgrade deal, but can they also change my phone plan? I’ve read some horror stories of Costco upgrades going wrong, so would it be better to upgrade the three phones on my account for the cash cards at Costco, and then change my phone plan at the ATT store?