How can I modify roles of an already existing user using Java API

Oh oh,  I know this.   What you have to do is get the user's current role(s), filter out the 'default' and append the new role[s].

The below method came from Ralph's 'dumpContributor' example which I shamelessly have made use of:

	private static IRole[] getProcessAreaRoles(ITeamRepository teamRepository,
	            IProcessArea processArea, IContributorHandle handle)
	            throws TeamRepositoryException {
	        IContributor contributor = (IContributor) teamRepository.itemManager()
	                .fetchCompleteItem(handle, IItemManager.DEFAULT, null);
//	        System.out.print(": " + contributor.getUserId() + "\t"
//	                + contributor.getName() + "\t" + contributor.getEmailAddress()+ "\t");
	        IProcessItemService processService = (IProcessItemService) teamRepository
	                .getClientLibrary(IProcessItemService.class);
	        IClientProcess process = processService.getClientProcess(processArea,
	                null);
	        IRole[] contributorRoles = process.getContributorRoles(contributor,
	                processArea, null);
	        return contributorRoles;
//	        for (int j = 0; j < contributorRoles.length; j++) {
//	            IRole role = (IRole) contributorRoles[j];
//	            System.out.print(role.getId() + " ");
//	        }
//	        System.out.println();
	    } 

	private IRole[] appendRoles(IRole[] newRoles,IRole[] oldRoles) {
		if (oldRoles == null)
			return newRoles;
		
		IRole[] roleList=new IRole[oldRoles.length];
		int i=0,n=1;
		roleList[0]=newRoles[0]; // this one should be a singular list
		while (i < oldRoles.length) {
			if (!"default".equals(oldRoles[i].getId()) ) {
				roleList[n]=oldRoles[i];
				n++;
			}
			i++;
		}
		return roleList;
	}

In the part of the Class that uses the above that actually adds/updates the Contributor:

				contributorRole[0] = findRoleByName(teamRepository, (IProcessArea)project, projectRole);
				if (contributorRole[0] != null) {
					if (userInRepos) {
						allRoles=getProcessAreaRoles(teamRepository,project,newContributor);
					}
				if (allRoles != null) {
					contributorRole=appendRoles(contributorRole,allRoles);
				}
				projectAreaWorkingCopy.setRoleCast(newContributor, contributorRole);