From the Game Manual
http://www.buzzerbeater.com/community/rules.aspx?nav=Rost... (emphasis mine):
When you link to a different team's roster from their home page, you'll see only each players weekly salary, age, height, and best position.
When Burks was drafted, the game took a look at his skill distribution and determined that the "best position" for his current skill set at that point in time was PG. However, you then proceeded to give him further training and improved his skill set from pops he received from training. Every time training is run, the game looks again at his skill distribution and determines what the "best position" is for his current skill set. Because of training he received (I am assuming he increased one of his ID/IS/RB/SB skills) the game determined that his "best position" to play him would be PF.
This "best position" is simply Buzzerbeater's way of looking at a player's skill set and making a suggestion about where they might fit into the team and where it might be best to play them. This is beneficial to opposing managers to help give an indication of what kind of skills a certain player might have. If I were to be scouting your team and see that Burks is listed as a PF, then that would be an indication to me that his inside skills on average might be higher than his outside skills. Additionally, since he is listed at PF and not C, that would imply to me that he has more outside skills than a player which would be listed as a C. As there are no benefits from or requirements to play Burks at his listed "best position" (which changed from PG to PF), there is no impact to your team's performance or training due to this change.