How difficult is it to change custody from joint to full custody at the wish of the child?

The degree of "difficulty" will mainly depend on the attitude of the parents, and the history of the case.  As a legal matter, a "child of suitable age and discretion" is legally entitled to have input into his living circumstances, and most judges do not want to separate siblings, so they tend to go together.  For the relevant statutes, a flowchart showing procedure, the listing of the statutory factors, and  lot more relevant information, see the explanation and materials posted at http://www.willicklawgroup.com/child-custody-and-visitation/.  But realisticaly, for a meaningful evaluation of youir chances of success, you should get copies of all relevant orders, information, and relevant documents, and make a consultation appointment with a qualified family law specialist to go over the facts and give specific advice.