You will have to litigate this issue anew in the divorce.
When there has been a major change in circumstances, issues related to custody can be relitigated, even if there isn't a remarriage.
In this case, both the marriage and the fact that nine years have passed since the original order constitute a substantial change in circumstances. At the time the order was entered, there was an 18 month old with parents who didn't live together. The child is now 10 years old and has lived with both parents without regard to any custody order for at least seven years.
These are completely different circumstances so custody issues must be revisited. As @mkennedy notes in the comments, it is even likely that the court or someone acting on behalf of the court would consider the opinions of the child at this point, which obviously wasn't possible the first time around.
Generally speaking, the way that the "best interests of the child" standard that applies in a case like this one is interpreted is to come as close to maintaining the pre-divorce status quo as possible in light of the separation of the parents.
The most relevant section of the Missouri Revised Statutes to this issue is as follows:
§ 452.410. Custody, decree, modification of, when
Except as provided in subsection 2 of this section, the court shall not modify a prior custody decree unless it has jurisdiction under the
provisions of section 452.450 and it finds, upon the basis of facts
that have arisen since the prior decree or that were unknown to the
court at the time of the prior decree, that a change has occurred in
the circumstances of the child or his custodian and that the
modification is necessary to serve the best interests of the child.
Notwithstanding any other provision of this section or sections
452.375 and 452.400, any custody order entered by any court in this state or any other state prior to August 13, 1984, may, subject to
jurisdictional requirements, be modified to allow for joint custody in
accordance with section 452.375, without any further showing.
If either parent files a motion to modify an award of joint legal custody or joint physical custody, each party shall be entitled to a
change of judge as provided by supreme court rule.