Why some place like Harvard? Graduate school rankings work differently than undergrad; you have to look at places that are ranked within your field. The rankings in engineering are going to be different from the rankings in anthropology, for example. (Far as I know Harvard's not highly ranked in engineering, but they do have a good reputation in anthropology.)
In any case, it is indeed possible to get a master's degree in anthropology and switch fields. A lot of people do switch fields by doing just that. Depending on where you live and how much money you've got lying around, it may be smarter to get a master's degree in anthropology from a less expensive school that still has a good reputation in the field and then apply to PhD programs at places that are better reputed in anthro.
What does "guaranteed graduate school" mean? I would take that very carefully, as few people are guaranteed even when their home departments look very highly upon them. Besides that, you don't select a graduate program based on convenience unless you're just earning a credential to make more money. If you're serious about academia or working in the field as an archaeologist/anthropologist, you need to pick a graduate program that suits you as far as the research of the professors and your own academic goals and interests go.