Daf Yomi - Yoma 42 - Kohen Gadol is a Kohen Hedyot Plus (only if he was once a regular kohen)

The Gemora states that the first porah adumah was done by Elozar, who was the sgan kohen gadol, but all future ones will either be done by a kohen gadol or a kohen hedyot. The Rogitchover Gaon explains why the first porah could not have been done by Aharon. The halacha is that even if a kohen gadol does the porah adumah, it must be done with the four בגדים and not the eight that the kohen gadol usually wears. This would signify that right then the kohen gadol is performing as a כהן הדיוט and not as a כהן גדול. The only way a kohen gadol can act like a regular kohen is if he was once a regular kohen, however אהרן הכהן was never a regular kohen - he was appointed a כהן גדול immediately and therefore he could not perform the פרה אדומה, for it was not possible for him to wear only four בגדים and be like a כהן הדיוט.

[There are those that say that every kohen gadol must be a sgan first - if so, aharon must have been an exception.]