All you need is the Add Party Member and Remove Party Member commands. Add them in the order you want. For example, if you had two party members:
Party Member A
Party Member B
And say you now want party Member B to be the party leader, you would say:
Remove Party Member A
Add Party Member A
At first the above may look like it does absolutely nothing, but it actually changes the order of the party so that Party Member B is now the party leader.
Hope this helps.