Assume a typical, database and session driven ASP application developed using best practices just before the first release of .NET. Assign the amount of effort required for seasoned professionals to implement it properly and efficiently to be 1.0.
What would you estimate to be the amount of effort required to maintain it using best non-aggressive refactoring practices from then until now?
Would that have been a good investment (Option A)? Or would it have been better (option B) to have done new work with current best practices, but no refactoring? Or (Option C) would it have been best to keep the architecture consistent by doing new work with the old architecture?
In light of your choice, what should you be doing from now on?
I don’t think you’re giving us enough information to give a sensible answer.
In general and all other issues being equal, I would always start with option C to make sure that decisions are driven by the business rather than by technologists.