Switching the Baal Korei in middle of the reading:

Initially one person is to read the entire Keriah of all the Aliyos, including Maftir.[1] This applies even if a second Sefer Torah is being read for Maftir.[2] However, in a time of need, one may switch the Baal Korei between Aliyos, such as if he is the set Baal Korei in the Shul and the current reader is not as expertise in the reading.[3]

____________________________________

[1] Shaareiy Efraim 3:6; Ketzos Hashulchan 84 footnote 15

[2] Poskim ibid

[3] Poskim ibid