In my opinion it's good practice always to record information in your facts exactly as it's written in source materials. Unfortunately, Family Tree Maker just doesn't handle situations like this as well as it could, so most people use what FTM offers as best they can. With that in mind, I might recommend deviating from the good practice here by including an interpreted place name with the correct spelling in your fact. The citation detail for the source should transcribe verbatim the spelling used by the author, and you should use a fact note for each one where you have interpreted Mindora to be Mindoro.
To change a place name en masse, do it from the Places workspace. To change facts into different facts, choose Manage Facts under the edit menu. There is no facility to duplicate selected facts en masse and change one to an alternate. You have to do that manually.