The documents that we use to support government are living documents and should be. Simply, how we felt about African-Americans in 1776 and in 1876 and in 1976 has been vastly different. Therefore, as we have changed as a society we need to make sure we have an organizing document that can reflect that change.
The Constitution is by design a document that can be changed. Article V allows for the amendment of the Constitution provided the amendment is ratified by three quarters of the states. This is obviously different from the phrasing in the question of 'changed at will.' Amending the Constitution is a very difficult process, and outside of the initial 10 amendments (the Bill of Rights), there have only been 17 amendments in the document's 225 year history.
Yes, I think both the Constitution and the Bill of Rights should be "living documents". The world was a much simpler place when those two documents were authored. I think our founding fathers meant them to be the basis for our new country. The world has changed over the years. Society has changed and we have many new situations that need to be addressed such as gender issues. These will require additions and amendments to both documents.
The constitution should not be able to be changed at will.What makes the US constitution unique is its ability to be changed or modified in an orderly process. Our system helps promote stability, other countries rewrite there constitution whenever they add/delete or changes. This add to potential corruption and mob rule type of governments. The US constitution attempts to protect the all the people equally and framework to change that at will would not be a wise course to take.