Is it permissible to always substitute the letter "seen" (س) with "saad" (ص) or vice versa in the Holy Quran, or only in specific cases?

Substitution is a common phonetic phenomenon in languages, explicitly mentioned by Arab linguists who provided reasons and parallels for it. This includes the substitution of 'س' (sin) with 'ص' (sad), especially when it precedes emphatic letters. Some Companions (Sahabah) wrote words like "المصيطرون" (al-musaytirun) and "بصطة" (bastah) with a 'ص' (sad), even though they could be read with a 'س' (sin), because the reading with a slight 'ص' sound (ishmam) is permissible. The general rule is that every 'س' (sin) followed by a 'ط' (ta), 'غ' (ghayn), 'خ' (kha), or 'ق' (qaf) is changed to a 'ص' (sad). As for the recitation (qira'ah), it is an established tradition not based on the most common linguistic usage but on the most authenticated transmitted tradition. It is not permissible to substitute 'س' (sin) with 'ص' (sad) in the Qur'an unless a sound narration has been transmitted to that effect.