The experienced engineer turns any constraint into an advantage. The advantage of an existing plant is that you don't have to theorize what it will do. You measure it. Accurate measurement of plant performance is the starting point of a succesful revamp. Most revamps are failures.
Calculations are assumed to be correct, it is reality that has it wrong.
Revamps start with data from the plant, not paper in the plant design data books. Cost decisions ignore operating requirements. Cost control is vital to project returns. However, items are often arbitrarily removed or changed to reduce cost without regard to the consequences on operation. When a project is in construction, the project manager is in charge.