Jamieson, Fausset and Brown on Ezekiel 21:26

26. diadem--rather, "the miter" of the holy priest (Ex 28:4; Zec 3:5). His priestly emblem as representative of the priestly people. Both this and "the crown," the emblem of the kingdom, were to be removed, until they should be restored and united in the Mediator, Messiah (Psa 110:2, 4; Zec 6:13), [Fairbairn]. As, however, King Zedekiah alone, not the high priest also, is referred to in the context, English Version is supported by Gesenius.

this shall not be the same--The diadem shall not be as it was [Rosenmuller]. Nothing shall remain what it was [Fairbairn].

exalt ... low, ... abase ... high--not the general truth expressed (Pr 3:34; Lu 1:52; Jas 4:6; 1Pe 5:5); but specially referring to Messiah and Zedekiah contrasted together. The "tender plant ... out of the dry ground" (Is 53:2) is to be "exalted" in the end (Eze 21:27); the now "high" representative on David's throne, Zedekiah, is to be "abased." The outward relations of things shall be made to change places in just retaliation on the people for having so perverted the moral relations of things [Hengstenberg].