You don't change it .... (you can dissable ultrapeer capabilities if you are having problems keeping a good connection) The Gnurtella network needs both and Limewire is configured to ensure the best configuration. I don't exactly understand it , but there is no advantage to being an ultrapeer, other than it sounds good, and it helps the network function
The following link to the Phex website gives a reasonable explanation of the subject. It is worth noting that Phex can be configured more exactly than Limewire, which would be an advantage to those who know what they are doing, and the opposite to those who don't
http://www.phex.kouk.de/mambo/content/view/28/38/