153. Taking Out a Second Torah
24:7 If a disqualifying defect requires that another Torah be taken out to be read, if the defect is discovered between aliyos, we resume reading from the place where we stopped and we complete the remaining number of aliyos. Those who already read from the defective Torah are counted among the suitable aliyos. If this happened on Shabbos, when we may add extra aliyos, and if there are enough verses remaining to be read, it's advisable to call up seven people for aliyos from the second Torah in deference to those authorities who would not count those aliyos read from the defective sefer.
24:8 If a disqualifying defect is found in the middle of the reading, there are different opinions and practices as to how to proceed; each community should follow its own established custom. Where there is no set practice, it seems that one should act as written in the Shulchan Aruch and not bless on the unfit Torah. (The Rema on OC 143:4 says that we do say a concluding bracha on the first sefer if more than three verses were read from it.) In such a case, the reading is immediately stopped and another Torah is brought; the reading resumes from where it was stopped. If the reading stopped mid-verse, one should resume at the start of that verse. We should read at least three verses from the kosher Torah; if there are not three verses remaining until the end of the reading, the baal korei backs up the necessary number of verses. The one whose aliyah was interrupted says the concluding bracha over the second Torah; he does not repeat the first bracha. If it is possible to call seven people for aliyos from the second Torah, that should be done. This is also the procedure when the defect is discovered after the bracha was recited but before the reading began, i.e., that the first blessing is not repeated. There is no difference between the seven regular aliyos and maftir.