If a Shul has a Klaf to lain the Haftorah from and there is a Bar Mitzvah in shul and the boy doesn’t want to lain from the Klaf only from a complete Nach . Is there any reason not to let him lain from the Nach and have the regular Bal Korah lain from the klaf ?
If the standard of the shul is that they are particular to specifically lein from a klaf, then the bar mitzva boy should practice so he can lein it from the klaf. This is the custom in many shuls in Eretz Yisroel- the boys learn to lein from a klaf, and the bar itzva boy should not come along and change the minhag of the shul for his convenience. However if the boy didn’t know, and he isn’t prepared for it, etc, and it is the last minute, then he may lein from the nach, so he won’t be embarrassed, and let down that he wasn’t able to lein