Symptom

You changed the Payer in the document from 'A' to 'B'. When the partner data is changed in a sales document, the credit data is not redetermined automatically. So the credit account (field VBAK-KNKLI) has still the "old" value, also the credit values remain for this account. 

Solution

In order to redetermine the "new" credit account for these affected documents, you will need to run report RFDKLI20, see note 408596. This report rebuilds the credit data after making organizational changes. Every document for the selected customers will be updated with regard to their credit data within an internal VA02. This means the document will be read, the credit data will be changed and then saved again. That means also the credit values of these documents will be updated to the "new" account, as if you would change the order within VA02.

Please read also: RFDKLI20 does not update new credit master data in SD documents