Forget about changing legal custody from joint to sole , instead concentrate on changing apportioned parenting time as it's not necessary to show a significant change in circumstances for those purposes . You should be able to seek more time for yourself especially if the court finds the child's wishes compelling .
2007) Change in circumstances need not be substantial for modification of custody to accommodate changes of scheduling parenting time between mother and father. Russell v. Russell, 210 S.W.3d 191 (Mo.banc).
-------------------- Never let your sense of morals get in the way of doing what's right. Isaac Asimov