So you need to formulate an application modernization decision -- what to do with a given application -- how do you begin that decision making process? In the past, modernization decisions were often simply declared -- "We are moving to this technology" -- for a number of reasons, such as, it:
Keeps us current on technology.
Provides a more acceptable user-interface or integration capability.
Increases our exposure to access by external customers.
Increases the volume of business transaction we can process.
Trades custom/bespoke applications for standardized application packages such as ERP, payroll, human resources, etc.
Fast-forward to today -- you could simply go with your gut -- declare a solution based on what you currently know (or think you know) about the application in question. But it's a new day baby -- a proposal like that, without proper justification, is likely to be met with one of two responses from management: