What subjects in a will are interchangeable among all states? Is guardianship the same no matter where you live? How about real estate? If I move frequently due to my company, is there anything in my will I know will always be valid or do I have to make changes every time I move? What are the big topics that change when you move?
Due to the full faith and Credit clause of the U.S. Constitution, which reads "Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State," your will executed in one state will be honored if you move to another state. So you don't have to get a new will every time you move. This is also true of revocable trusts; they will be honored in all states.
This is less true of durable powers of attorney and health care directives. While they should be honored from state to state, sometimes banks, medical professionals, and financial and health care institutions don't accept documents and forms with which they are not familiar. In addition, for some purposes the execution requirements may be different. You ask about real estate. Some states require witnesses on durable powers of attorney and others don't. A state requiring witnesses may not allow a power of attorney without them to be used to convey real estate even though the document is perfectly valid in the state in which it was executed.