No back story required. Software can always be measured financially. The success of a rewrite should be measured in financial terms compared to an expected cost

No back story required. Software can always be measured financially. The success of a rewrite should be measured in financial terms compared to an expected cost target. If you have good test coverage prior to the rewrite the cost of rewrite will be substantially less because there exist many fewer unknown success and failure criteria. If a thorough plan and requirements strategy exist in writing before the rewrite starts the cost of rewrite will be substantially less because there is less exploration necessary. If you have a good type system in place, such as interfaces defined or a means to define them rapidly without a regression, the cost of rewrite will be less because the technical definitions are already defined. A rewrite is going poorly when it costs more than plan. A rewrite is going well when it costs less than plan. A rewrite is phenomenal when it costs less than plan and eliminates future maintenance expenses.