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Perhaps I do not quite understand what you are proposing, but if the Plaintiff and Defendant settled a matter, the defendant cannot file a new suit to relitigate the same matter. (They are barred by the doctrines of "res judicata" "issue preclusion" and "claim preclusion"). There is a very narrow exception for those instances where a party has concealed facts through fraud, but this is a very narrow, and seldom successfully litigated, exception.
What your post suggests is more closely resembling a case where the defendant simply decided to settle the case in an effort to avoid the cost and uncertainty of litigation - despite the fact that the plaintiff had a weakly supported case. This is not a basis to reopen a matter.