First of all, I would not want to touch Arrears, due to how functions PRDNT and PRPRI process data, and due to retro processing. If you need to adjust values in arrears table, it means something went wrong via configuration?
I would not adjust the value in the ARRRS table, try to adjust the value of the WT outside the arrears processing, via a custom rule and possibly a new adjustment WT that you would link to the WT to be adjusted, in T51P1. The rule will use operation r51p1 to generate the adjustment value in the WT you need to change, but outside arrears, and try not to do this in a past period (retroactively calculated)