Wa alaikum Assalam, Respected Brother!
Please remember that I am only a humble, fallible student of the Qur'an. Anyone can disagree with my thoughts or conclusions and I am ever ready to correct my views if anyone points to an error with reason through the Glorious Book.
The Qur'an honors the 'grandfather clause', namely, whatever has happened has happened. I have never said that a second wife must be divorced.
AN EXAMPLE OF THE GRANDFATHER CLAUSE:
4:22 Do not marry women whom your fathers ever married except what has already happened. Doing such was certainly a shameful and abominable custom.
4:23 The following women are prohibited for you in marriage: Your mothers, daughters, sisters, father's sisters, mother's sisters, brother's daughters, sister's daughters, foster-mothers who have ever nursed you, foster-sisters, your wives' mothers, your step-daughters, unless you have divorced their mother without being intimate with her. Also forbidden are women who have ever been the wives of your sons. You are not allowed to keep two sisters in wedlock at one time except what has already happened in the past. God is Forgiving, Merciful.