In parshat Lech Lecha, (Bereishit 12:3) Hashem promises Avram that וַאֲבָרְכָה, מְבָרְכֶיךָ, וּמְקַלֶּלְךָ, אָאֹר, And I will bless those who bless you, and the one who curses you I will curse.
Why does the language of curse change from k-l-l to a-r-r? In other places such as Ber. 27:29, the same k-l-l word is used and in Bamidbar 24:9 the a-r-r root is repeated. Why mix them here?
While I have found one reference (on Shmot 22:27) which explains a difference between the two
Kalal is a curse that someone should lose his status, while arar is one that he should dry up and not have any blessing. Kalal is motivated by defiance, while arar is motivated by envy (Hirsch; Chothem Takhnith, p. 125). Some say that arar is a curse that one should be cast down (yarah) or shut out from divine light (or). (Yerioth Sh'lomo, Volume 1,3:13, p.88c).
This does not explain why one would change words in the middle. I checked many of the meforshim and they mostly discuss the order of curse/bless so I haven't seen anyone explain the mismatch of words.