I was wondering if Roberts could remain a justice but the office of “Chief Justice” could be given to one of the other justices....would the senate have to concur or could Trump just do it?
No. It’s a specific office. If they wanted to move Roberts in favor of another (say, Thomas), each would have to go through the confirmation process again.
Roberts was a regular justice until George W. Bush nominated him for Chief Justice.
I’m not sure if Roberts can resign the Chief Justice position, yet stay on the court when the court is full. That would require the president to choose the new Chief Justice from the judges currently sitting on the court. The president might not want to appoint, and Congress might not want to confirm, one of the sitting associate judges. In that case it would seem Roberts would have to leave the court.