What is the ruling regarding a formerly non-Muslim husband marrying women previously prohibited to be joined with his existing wife after conversion to Islam?

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If a Magian or idol-worshipping husband converts to Islam, or if the marriage is dissolved through Khul', suckling (Rada'ah), annulment due to defect or inability to pay maintenance, or other reasons, the husband is prohibited from marrying any woman whom he could not lawfully be joined with his wife concurrently until the wife completes her waiting period (Iddah), regardless of whether the separation is deemed immediate or not. If the wife converts to Islam and he marries her sister during her 'Iddah, and then both of them convert during the first wife's 'Iddah, he must choose one of them, as if he had married them simultaneously.