How did Thomas Jefferson's political beliefs change over time?

Before becoming president, Jefferson favored a weak central government and a strict and limiting view of the constitution. He wanted the common people to hold the political power, and favored progress through trial and error. He was pro agriculture annd was against having a National Bank and standing armies.

When he became president, he pardoned the prisoners of the Sedition Acts (they stated that anyone who impeded the polices of the government or falsely defamed its officials would be liable to a heavy fine and imprisonment) and did away with the excise tax on whiskey. Actions like this show that he did stay true to his values to an extent.

However, when he made the Louisiana purchase, he did technically betray his idea for a strict interpretation of the constitution, because the constitution didn't specifically give him that right. He excused the action by saying that it was a treaty with France, and he had the right to make treatys, which somewhat mollified his political party. Jefferson knew that the Louisiana purchase was too good to pass up, but he overstepped his own ideals in buying the land.

How drastically Jefferson changed is largely a matter of opinion and dependent on which examples you choose to examine and how you interpret his actions. It is probably most accurate to accept that there is justification for both arguments and therefore say that Jefferson changed his political views moderately, possibly because his political view was too idealistic to be plausible.