It is possible to say that Jefferson’s actions with regard to the Louisiana Purchase were incompatible with his previous ideas because he previously believed that the Constitution of the United States should be interpreted very strictly. Since the Constitution does not say the government can buy land from other countries, a strict constructionist should not have been willing to make the purchase.
In the early United States, there were two points of view on the powers of the federal government. Some people, most of whom were Federalists, believed that the federal government should have a great deal of power. They believed that it should be able to do anything so long as the Constitution did not forbid it. It was this reasoning, for example, that led to the creation of the Second Bank of the United States in 1816. By contrast, the Democratic-Republicans believed that the federal government’s powers should be rather limited. They believed that the federal government should only be allowed to do things that the Constitution explicitly said it could do. Jefferson was one of the leaders of the Democratic-Republicans.
The Constitution does not explicitly say that the United States government can buy land from other countries. Therefore, Jefferson should have been opposed to buying the land without a constitutional amendment allowing a purchase. (And, in fact, Jefferson wanted to push for an amendment but eventually came to realize there was not enough time to do so.) Therefore, we can say that his purchase of the Louisiana Territory from France was incompatible with his beliefs about the Constitution.
The primary debate during the early years after America's establishment was regarding the proper size of e federal government. Some advocated a stronger federal government due to the failure of the Articles of Confederation following its lack of ability to properly bring the states together in issues of national defense and the taxes. others, like Jefferson, stood on the states' rights because America had just fought a Revolutionary War against a country whose federal government had bee too strong. As such, Jefferson (prior to the Louisiana Purchase) acted to follow his belief in the strict interpretation of the Constitution.
However, nowhere in the Constitution did it state the President was allowed to use the country's money to expand the size of the nation. Yet, Jefferson understood the value of the purchase and the extreme bargain the purchase represented. As such, he turned on his prior ideals in order to take advantage of this rare opportunity.