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?
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.