In SLII, there is a recognition that leadership is "done with people, not to people." The change in words illustrates this change of philosophy accurately. In a sense, the two models have slightly different conceptualizations to reflect changes in the audience. Situational leadership was developed by the authors in 1972, when leadership styles were far more autocratic than they are today. Leadership was done to people in those days. Today, leadership is done with people. . The authors have recognized this fundamental shift in the prevailing leadership attitudes and made the appropriate changes to their model to reflect this. However, the terminology used in the original situational leadership model will still appeal to autocratic leaders. The SLII model's terminology will appeal more to modern leaders. As the excessive and clumsy use of the registered trademark symbol throughout their paper indicates, these concepts are products being sold. A shift in wording to update a product and make it more appealing to a modern audience is clearly...

The axes remain the same (high, low, supportive, directive). Since the fundamental role of leadership describe in the model has not changed, only the ways in which a leader might implement those fundamental roles. So SLII can be viewed more as an update to the original model rather than a wholesale change. Certainly, the model works the same way, and roughly describes the same functions. As such, the natural conclusion is that there is not much difference between situational leadership theory and situational leadership II.
Works Cited:

Hersey, P., & Blanchard, K. (1988). Management of organizational behavior: Utilizing human resources. Englewood Cliffs, NJ: Prentice Hall.

Hersey, P & Blanchard, K. (2009). Situational leadership & situational leadership II: Commonalities and differences. Self published. In possession of the author.