What does "Begadim Acherim" mean?
Rashi: It means inferior Bigdei Kehunah.
Ramban, Moshav Zekenim: Some say that they are Bigdei Chol. 1
Since it is not considered an Avodah - Indeed, even Ba'alei Mumin were eligible to take the ashes outside (Ba'al ha'Turim).
Why must the Kohen who removes the ashes from the Mizbe'ach change his clothes?
Ramban: The Kohanim had two sets of clothing, one better quality (newer?), the other, inferior quality - and it is a Mitzvah when taking out the ashes to change into the latter (for the reason cited by Rashi).
Which ashes is the Pasuk referring to?
Rashi: It is the ashes that were piled on the 'Tapu'ach' (the pile of spent ashes that had been removed from the Ma'arachah). 1
When it became excessive (Rashi).
To where did he take the ashes?
Rashi: Refer to 4:12:2:1.
Why does the Torah find it necessary to add "el Makom Tahor"?
Rashi: Refer to 4:12:3:1.