The stated purpose of the fiance visa guide and program is to enable the alien fiancee of a U.S. citizen to enter the United States for the SOLE PURPOSE of marrying her petitioner. The K-1 visa is valid for 90 days and during that period the fiance beneficiary must either marry the American petitioner or depart the United States. Under no circumstances can a fiancee visa beneficiary marry someone other than her petitioner or change status to any other type of visa.
The USCIS is very concerned to ensure that a foreign fiance does not enter the United States on a K-1 visa sponsored by one American man and then marry a different American man. In 14 years of practice, I have run into this precise situation a couple of times and there are no options for the foreign beneficiary other than leaving the U.S.
The language of section 245(d) of the Immigration Act clearly precludes fiances from adjusting status on any basis other than marriage to the fiance petitioner. See Markovski v. Gonzales, 486 F.3d 108, 110 (4th Cir. 2007) (stating that the language of section 245(d) of the Act is clear insofar as it bars K-1 visa holders from adjusting status on any basis other than marriage to the fiancé petitioner); see also Kalal v. Gonzales, 402 F.3d 948, 951 (9th Cir. 2005) (same).
Over the years the USCIS has been very concerned to ensure that foreign fiancees are motivated by love and commitment and not the prospect of American immigration. One of the safeguards the USCIS has put in place is that the foreign fiancee is only able to obtain conditional permanent residency for two years after marrying her American petitioner. At the end of the two years, both the alien wife and American citizen husband must jointly petition to remove the conditions on the permanent residency and extend the permanent residency for 10 years.
The only exception to the two year rule is if the alien wife can show that she entered into the marriage to the American petitioner in good faith and has been abused by the American petitioner. In that case she can file the I-751 petition herself with evidence of the abuse.
In my experience the USCIS is genuinely trying to look out for the interests of American petitioners by ensuring the genuineness of foreign fiancees.