Under full faith and credit, the local law of the state of rendition will be applied to determine whether the judgment is modifiable and, if so, in what respects. This law will determine whether the judgment is modifiable with respect to past due installments and with respect to future installments. As between states, full faith and credit requires application of the local law of the state of rendition to determine whether the judgment is modifiable and, if so, in what respects.