Unable to change Biography style in Referencing - (Word in Office 2016)

I have written a book in four sections, the first two sections have IEEE set as the referencing style, but for some reason, the second two are set at ISO 690 and I can't change it.  What should be a simple click to change from one style to another does not work, and if I try I get an 'invalid source specified' in place of all my citations.  I have tried cutting and pasting the two offending sections into a new document (including citations) and then setting up the Biography, but no luck.  Help!!!!!