Is it ok to wear undertops and pyjamas during the 9 days that you didn’t ‘prepare’ beforehand?
According to a many poskim they should be prepared before wearing them. Many people rely on the poskim that say that it isn’t needed, however it is prefered to prepare them beforehand. Even if they weren’t prepared before the nine days they can still be prepared during the nine days by placing them for a little while on an unswept floor so they can get a little dirty, so they will no longer have a fresh clean feel to them. Alternatively they can be placed in the laundry hamper for a few minutes so they lose their fresh smell and feeling. A third option is to wear them on Shabbos, even if you will change a few times, then after Shabbos, they are no longer freshly laundered
There are 3 different opinions regarding clothing meant to absorb sweat.
Some poskim say that they are the same as other clothing (Kaf hachayim 551-91, Ohr L’tzion 3-2791), Kara Olay Moed 4 ftnt.4 in the name of Chut Shani, Alei Siach pg. 162 in the name of R’ C. Kanievsky shlit”a.) If however they were prepared beforehand they can be changed.
According to the poskim that one should only change them if needed. (Minchas Yitzchok 10-44, The Three Weeks pg. 84 ftnt. 139, quoting R’ S. Felder shlit”a in the name of a number of talmidim of R’ Moshe Feinstein zt”l, including R’ Reuvain Feinstein shlit”a. Ashrei Hoish O:CH 69-49, Nitei Gavriel 34-5). According to them, preparing them beforehand would also allow them to be changed. Also see Teshuvos Vhanhagos 5- 170, Yerushalayim B’moadeha pg. 201, that it is prefered to change them beforehand.
According to the poskim that hold that it is permitted to change these types of clothing even without a need, (M’bais Levi 13 pg. 27, Kovetz Halachos 11-23, Avnei Yoshpe 3-58 (23), Divrei Chachomim 459) they hold that there is no need to change them beforehand.