Question about upgrade and migration.

                Hi.So I'm a current Sprint customer looking to upgrade my phone early (still owe around $150), since Sprint and T-mobile have merged I thought I could choose a phone from the Tmobile site that doesn't show up on the Sprint site, but whenever I log in to anything on Tmobile it kicks me back to Sprint where that phone isn't available.  If it helps, it is the OnePlus 9 Pro . The Sprint site only has the normal version. Also Would I have to switch my account fully to Tmobile to  get that phone  ?