What is the value of current practices and infrastructure as they operate in processes & systems today to support the target digital models? The value really depends on the nature of your transformation. If when you established you target business and technical architectures there were radical departures from what is being done today, then the current architecture becomes more important. If the target architecture is a quite different and shows great innovation or there is desperation to move away from an old business model, then the current architecture becomes only a reference to check for making sure the target architecture doesn't miss important ingredients.
See http://jimsinur.blogspot.com/2015/03/targeting-your-new-digital-presence.html for some important targets to hit in the new emerging digital world.
When the Current State is Imperative:
If your organization has invested heavily in legacy purchased or bespoke processes/systems and there is not a significant threat in your industry, then salvaging the current processes/systems is important. You can still enter the digital world by changing the customer experience with creative mobile uses, supercharging decisions with big data analytic approaches and cutting costs with cloud based cognitive computing. One can employ and incremental transformation approach to getting to a better digital business over time.
When the Current State is Advisory:
If your organization is currently or expected to be under competitive duress from up-start or current competitors, or governance demands are dictating radical solutions or your executives anticipate new opportunities/threats, then the emphasis in on getting to the target digital platform quickly. In these situations the current processes/systems have a diminished value and should be used as a reference model to determine important missing pieces, policies or practices. It might be fine to then blow up the current processes afterwards, but at a minimum conservative organizations like to run in parallel for a period of time.
There is great danger in over valuing the current state in that you can cut off real innovation and find an excuse to aimlessly evolve. There is also a great danger in throwing out the investments and experience from the past and receive the hard knock lumps all over again. There us a real balance to strike here. I have lived through both versions of these mistakes, but we always arrived at the right answer over time. The problem is that today's business environment does not allow for that time to steer back to the right course unless incremental approaches are employed. Balance is the watch word, but keep in mind that the resistance to change will bias towards current state.