I have 2 children, ages 13 and 9. I have been divorced for 9 years and the children have been in my custody for the past 9 years. In those years, we (kids, step-father, me and 3 year old sister) have bought a house, have been attending the same school for over three years. The children have been getting good grades in school and are active in extra curricular activities. The children and I live in Arizona, and the divorce was done in California( ex lives in CA ). Since the divorce,I am always very welcoming and supportive of making visitation happen, I never went through the courts to seek more child support, even though his current job pays a whole 90k more than when we divorced.
Now he wants the kids to live with him, and they don't want to go because of the lives they have established in Arizona, they are happy here. ( ex and I share Legal custody but I have sole physical custody) what is the likelihood of him getting custody?

Slim and none in my opinion. For a court to order a change in custody, there generally has to be a problem with the current custodian. And that doesn't appear to be the case.

The problem you have, though, is CA still has jurisdiction. So any hearings would be in CA courts.

Thank you!