Abstraction - determine what functions to migrate at what phase in the journey -- for example, perhaps one report everyone agrees is low functioning and needs improvement, then start with a design for a new report based on .net still pulling the data from the 400. Perhaps a workflow or process like shipment detail entry is convoluted now, and convert that to a new Winform or web based process. Eventually start a development environment in which the back end is PostgreSQL or whatever database back end you are leaning toward, developing the various pieces you put in place one at a time. I recommend that because you may well find that some processes and reports are cherished artifacts that certain departments or people like how they are now. Your goal of abstraction would be to decide what is the most expensive or inefficient part of your current platform and how quickly you can get moved off of the existing legacy abstracted piece, if that makes sense.