A "magistrado" is in effect a senior judge in that it means a judge of a collegiate court, rather than a single-judge court, and the former are generally higher courts.

"magistrado s
1 (Der) Funcionario judicial que forma parte de un tribunal colegiado: magistrado del tribunal superior"
http://dem.colmex.mx/moduls/Default.aspx?id=8

You could reflect this by putting "a judge" or "one of the judges".

I would be inclined not to use "change of judge", since in the US this strongly implies recusal of a judge in a lower court. We don't know whether this was recusal, misconduct or indisposition. I think "replacement" would be better.

As to whether to use "judge" or "justice", I think you could do either. In the SCOTUS, the judges are called justices, and also in some state supreme courts and federal appellate courts, but not all. However, on balance I think I would probably use "justice" in a translation for the US.

The following document, on Massachusetts appellate procedure, uses both:

"170
(b) Replacement of Justices. If a justice who has participated in a case becomes unable to participate further, then the Chief Justice of the appellate court may substitute another justice."
"This text allows for the replacement of a justice should that justice become unable to participate in the case.
"The new text is more clear and allows for the replacement of a judge should that judge become unable to participate in the case. This is current practice, and these amendments make it clear that judicial substitutions may be made as needed."
http://www.mass.gov/courts/docs/sjc/invitations-to-comment/p...