Modification, exchange, or substitution.
Commutation is the replacement of a greater amount by something lesser. To commute periodic payments means to substitute a single payment for a number of payments, or to come to a "lump sum" settlement.
In CRIMINAL LAW, commutation is the substitution of a lesser punishment for a greater one. Contrasted with clemency, which is an act of grace eliminating a sentence or punishment, commutation is the modification or reduction of a punishment.
The change from consecutive prison sentences to concurrent sentences is a commutation of punishment.