First, if you have a Roku TV you don't have to use a Roku player on it. Both the Roku TV and the Roku player will have the same apps available. Depending on the TV the Ultra could have better performance, but that would only be important for browsing through the channels/apps and maybe a sightly faster loading time for a selected app. Beyond that they're pretty much going to have the same experience.
Can you use a Roku player on a Roku TV? Certainly, no restriction on doing that. But it can be confusing with two different Roku remotes that only control their individual devices.
You would activate the Ultra the same way you would any new Roku device, by linking it to your existing Roku user account.
Dan
Roku Community Streaming Expert
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