Indeed, most scholars assert that Jefferson himself was inconsistent, and that his views changed radically over his lifetime. True, Jefferson himself did not write a systematic book of political theory. On the contrary, I wish them known and therefore willingly express them to everyone. Jefferson to S. Smith, 1798. It was not the principles, but the times and circumstances that changed.
Different circumstances require the application of different principles.
In fact, there is a danger in becoming wedded too much to the historical circumstances in which a principle is enunciated, and in losing sight of the great and noble principles involved by ascribing them to temporal expediency. If something is truly a principle, it remains so regardless of the context or circumstances. Or, perhaps more accurately, Principles are not restricted to a single context, but apply to many different situations. Different circumstances may call into action different principles, but if Jefferson enunciated a principle in reaction to, for example, the Napoleonic conquests, then that principle remains true for any other situation where the relationships and events are essentially the same. The date when a statement of principle was made may be important, because it helps us to understand the circumstantial context which made the application of that principle appropriate.