Sforno on Numbers 20:26/ספורנו על במדבר כ׳:כ״ו

1והפשט אהרן את בגדיו, the garments that were exclusively those of a High Priest, i.e. those in addition to the garments worn by the ordinary priest.

2והלבשתם את אלעזר, who was already wearing the standard four garments which every priest wore. As a result, Aaron would remain wearing the basic four garments worn by every priest. It is precisely these four garments which he would wear any time he had occasion to enter the Holy of Holies, so that he in no way was deprived of status by not wearing the garments which were meant only for his image among his peers, not for his image when facing G’d. In fact, basically, what he remained with were the kind of garments angels appear in when they appear to human beings.