In yet another twist in its five-month-old pursuit of Yahoo, Microsoft said Monday that it would be interested in negotiating a purchase of Yahoo’s search business or of the entire company, as long as Yahoo replaces its board. Mr. Icahn is mounting a proxy fight against Yahoo’s board, one built on the gamble that he can bring Microsoft back to the negotiating table. But without a commitment from Microsoft to complete a deal, let alone any discussion of price, it is unclear whether Microsoft’s a