"I do not believe in packing the court," Sanders said during the second of the first two 2020 Democratic presidential debates. "We’ve got a terrible 5-4 majority conservative court right now. But I do believe constitutionally we have the power to rotate judges to other courts and that brings in new blood into the Supreme Court and a majority I hope that will understand that a woman has a right to control her own body and that corporations cannot run the United States of America."
For background, Article III, Section 1 of the US constitution is in full:
The judicial power of the United States, shall be vested in one Supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish. The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.
Article II, Section 2, states in part:
[The President,] by and with the advice and consent of the Senate, shall appoint ... judges of the Supreme Court, and all other officers of the United States, whose appointments are not herein otherwise provided for...
Is Sanders' claim (in bold above) correct? Does the US President, or anyone else for that matter (other than a Supreme Court justice choosing to retire from their position on the Supreme Court) have the power to "rotate" those judges to other courts?
Consistent with Sanders' claim, this question is asked under the current US constitution assuming no amendments; just about anything is possible via amendment.
Also, while a bunch of folks on Law.SE might be able to speculate or find persuasive arguments posted in answers below, who would be able to authoritatively decide the constitutionality of such a question, with all Supreme Court justices having clear conflict of interest on the matter?
A couple law professors posts proposals here (and in more detail here, reviewed here), one for a rotating panel of appeals court judges to serve on the Supreme Court, and another for a 15-justice Court with 5 appointed by each party and 5 picked unanimously by those 10, without the ability to hear cases if those seats are empty. There is also a proposal circulating for 18-year term limits followed by remainder-of-lifetime service on a circuit court. If any of those, or other strategies, appear constitutional at present, an explanation of why would be a great answer, as would be an explanation of why any proposal involving rotation consistent with Sanders's quote would not be possible under the current constitution unamended.
Note: The question framing intentionally excludes consideration of any person or military force who might have the physical firepower to remove Supreme Court justices in a way that would leave them unable to serve as a judge in another federal court. That's not "rotating" off.