> Why did Jefferson change Locke's "life, liberty, and property" to "life, liberty, 
> and the pursuit of happiness" when he wrote the Declaration of Independence?

Jefferson did not provide an explanation for substituting "the pursuit of happiness" for
Locke's "property."  Locke's second treatise was well known to the Founding Fathers, of
course.  But we can only surmise why they readily accepted this change.

The first part of the Declaration of Independence was intended as a philosophical
foundation that would demonstrate the true and ultimate meaning of government in order
to justify the colonists for taking the steps that they were taking, i.e., to break off
their connection with Great Britain.  Jefferson, therefore, sought to define government
in a way that captured the best concepts then prevailing.  His intent was not merely to
justify independence, but to transcend the ordinary concerns of the moment and make
what might be called an ultimate or timeless definition of what government means.
Then, starting with that ground, he would work up to explaining how and why these
fundamental principles had been violated, giving rise to the absolute necessity that the
colonists take whatever steps they must in order to remedy the present situation, i.e., 
to break off their relationship with Great Britain.

It is obvious that no justification for a separation from the existing governing authority 
can be made unless it can be shown that the ultimate powers of government derive from
the people themselves.  If the restraints on government derive merely from the demands
of justice and of whatever is necessary to make both government and society work, such
theoretical demands could only argue for a reform in government, not an overthrowing
of the existing governmental powers.  On the other hand, if government actually has
a responsibility to its people, and if it fails in that responsiblity and is seen as 
being destructive to the life of its people, then the people have the natural right to 
overthrow such government and install in its place one that is more conducive to their 
safety and happiness.  It is by this means that Jefferson was able to justify the 
American Revolution.  This is precisely what the Declaration of Independence 
demonstrated.

Locke's writings supported a government FOR the people, but not a government OF and BY
the people.  Locke argued for just government, but his ideas of just government merely
served as a constraint upon kings and parliamentary powers.  They did not serve to
establish the principles of popular self-government -- government where the people
themselves are the ultimate authority, above both king and parliament.  That concept of
government was in advance of Locke and his contemporaries.  To Locke, the government 
was obligated to respect the rights of its people, but the people themselves were not
looked upon as the ultimate sovereigns to whom king and parliament were responsible.

Jefferson and the Founding Fathers had different ideas, however.  They recognized in
the people themselves a dynamism that should be free to express itself through government,
and therefore they sought a philosophical foundation that was more profound, more
fundamental, than mere justice and its constraints upon established governing authority.
Where Locke recognized the rights that government must respect in order to have a
properly functioning society that fully served the needs of social development, Jefferson
recognized the power of the people that must shape and determine government in order that
individual members of society shall be able to experience their optimal functioning as
individual human beings.  This for Jefferson meant the sovereign powers of government 
must flow from the people, and that their consent was necessary in order to fulfill this
ideal.  The people were to make choices respecting government that would conform with
what THEY thought were needed in order to fulfill this objective.  Then they would watch
government carefully to make sure that it conformed to their choices.

Therefore, when Jefferson elucidated the basic rights of man, he went beyond Locke's "life 
liberty, and property," and stated instead, "life, liberty, and the pursuit of happiness."
Locke's basic rights are only aimed at justice.  Jefferson's are aimed at human fulfillment.
Locke's tell the governing authority what they must do.  Jefferson's tell them how they
must constitute themselves and who must have ultimate controlling authority over them.  It
is an entirely different approach to the meaning of government.  Locke would tell the king
what he should do.  Jefferson would tell the king that he has no place in a free society.
Locke would empower wise men who would exercise intelligent supervision over the government
and society, and would make decisions based on their best thinking.  Jefferson would
empower the people themselves to exercise wise oversight and carefully watch the 
persons they appoint as governors to make sure they conduct themselves properly.

Locke listed his essential rights as "life, liberty, and property" because they are the 
basic entitlements that individual human beings must have to be considered free men.  
When Jefferson went beyond property and used the term "the pursuit of happiness," he 
used a more comprehensive term.  This does not mean that he did not consider property 
rights to be essential to man's life in society.  The pursuit of happiness implies not 
only the right to own property, but any and all other rights necessary for the optimum 
development of human beings, such as freedom of speech, of religion, of conscience, etc.
In fact, the pursuit of happiness as an inalienable right is a timeless human necessity
that speaks to the human condition, but is subject to reinterpretation in every 
generation, allowing for the constant redefinition of the pursuit of happiness as 
befits the changing circumstances of a changing world.  Locke's much more narrow 
concept, while certainly valid as far as it goes, is nowhere near as flexible and 
expansive for later generations as is Jefferson's formulation.  Compared to Jefferson, 
Locke does not capture the vision of the fullest possible human growth and development.
 
Although Locke's ideas certainly had great influence upon Jefferson and the Founding
Fathers, Jefferson went far beyond Locke when he established the basic principles 
of American self-government.