To anybody wanting to move to Agile/Lean without having an Agile/Lean mindset, I would recommend taking today’s pill of common sense from Lean Software Development: An Agile Toolkit, by Tom Poppendieck and Mary Poppendieck.
“The notion that a design must be complete before it is released is the biggest enemy of concurrent development. Requiring complete information before releasing a design increases the length of the feedback loop in the design process and causes irreversible decisions to be made far sooner than necessary. Good design is a discovery process, done through short, repeated exploratory cycles.” – excerpt from Chapter 3. Decide as Late as Possible