The mishna, Tamid 5:3, indicates that those kohanim who had gotten dressed for the day's work but were not needed for it would be stripped of their kohen-clothes by the Temple workers.
My kid asked: Why didn't they undress themselves?
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It seems like the Tifferes Yisroel (Yochin 14) there understands this to mean that the Temple workers were there to take the clothes back from them and return them to storage - and not that they actually physically removed the clothes from their bodies.
At least one of the garments required assistance in dressing/undressing - the אבנט (the belt), which was 32 amos long (more than 48 feet)! Also, the winding of the head covering would probably also require assistance.
The Mishnah in Shekalim 5:2 mentions "Pinchas the Dresser", and the gemara Yerushalmi Shekalim 22b says that he dressed the Kohen Gadol.