No, if you intend taking up permanent residence in the U.S., you are required to wait until the immigrant or fiance visa is issued. You cannot reside in the U.S. on a tourist visa or visa free under the Visa Waiver Program while waiting the issuance of an immigrant or fiance visa. When traveling to the U.S. either with a visa or visa free under the Visa Waiver Program, you should be sure to carry with you for presentation to U.S. immigration evidence of your residence outside the U.S. If the BCIS inspector is not convinced that you are a bona fide visitor for pleasure, you will be denied entry into the U.S.
No, you may not change status to K-1 while you are in the U.S. The K-1 visa is solely available to finace's of U.S. citizen's outside the U.S. to enter into the U.S. and marry the U.S. citizen.