upgrade device after keep and switch

I am new to t-mobile and I did the keep and switch promo; I was told I am able to upgrade my device as soon as I receive the e-gift card and pay off my devices with my previous carrier. 

If you upgrade before that is done you risk losing the promotion and they potentially can ask for repayment of the money (no idea why or how that is, but I was heavily warned to not follow directions). 

While waiting for the e-gift card to arrive (typically 15-30 days) doesn’t seem like long, its an eternity when your new service is terrible and the t-mobile store tells you it possibly will improve once you have a device that they sell, but you have to stick it out in the meantime. 🙄Frustrating, and ridiculous, but here is to hoping it works!