So I got it...
My string has Normal style but different font (Normal style has Times New Roman font). So the string actually has not only a paragraph style but a charatcer one too. So I need select all the text (Ctr+A) then change the whole document's style to Normal. It changes the font of the whole document to the default Normal style's font. And after that, the 'Replace' method works well and changes the replaced string's font to Cambria.

Nice day!