I can change your culture in 5 minutes.  It's commitment and consistency that take time.  It's overcoming the disbelief and old attitudes.

The first thing we do wrong when trying to change culture is to plan it and approve it.   Culture change requires, like good software, continuous delivery of new value.   The most important thing to get right is inspiration.  Next, the implementation has to be swift and small BUT it must be driven by passionate, focused individuals.  It can't be left to those who are 50% convinced.

Small teams driving cultural change will be more comfortable with experimentation and failure.  So if their work doesn't catch fire, they are undaunted.  What matters is the effort of change.  Not a specific output.

Spreading change is a difficult and uphill push.  It is THE leading cause of agile failure.  What is required here is internal certainty and consistency.