Role and responsibility of executives in a lean/agile transition

1) Choose internal change agents carefully

Volunteers
Positive mind and change orientated
Sense of urgency
Capacity for reflection
Work level experience in software development
Transition team covers different project roles and organizational units
Involve line management
Involve product management / business people
Involve process / Quality Management early

2) Be sponsor to the transition team

Give your vision, or agree explicitly to vision and mission of the transition team
Tell them what the constraints are
Be interested in plans and results.
Reed at least one good book yourself that does not look like a Scrum Bible, but something explaining what may work and why.
Do not expect to have NO impact on project time lines, this is just wishful thinking.
Give a reasonable budget based on estimates of the transition team
Do not tell them how to make the transition. Rather be able to discuss organizational changes on their request.
Let the team choose external consultants, but ask these people a couple of tough questions before you contract them.

3) Take care about trust in your organization

Do you, and do your middle managers trust your teams to get their job done? This is an essential part of any lean/agile transition.
If you are still not there, expect transparency to grow at the project team level, but you have to inspire trust from the management level. You need to start! You need to express trust to your teams and act accordingly yourself.

4) Do not combine a lean/agile transition with negative measures

Especially, do not try to reduce headcount. This is directly counterproductive to encouraging people to share their knowledge.

5) Foster a learning organization

Expect your organization to be on a learning journey for more than the transition lasts. The agile principles expect you to hire the best people, and give them the environment so they can be productive. In traditional hierarchical companies, often people need to learn to learn again. And often the environment consists of crappy old tools that make them slow. You may have tons of legacy code without automated tests. They will not magically disappear at the agile transition. The teams have to learn and improve a lot until they will really feel agile.
You will see improvements very soon, but this is not the end of your journey.

6) Look for opportunities to benchmark your transition with others

As executive you should use your connections with your colleagues from other companies for exchanging with them and creating opportunities for the transition teams and different project roles for comparing and exchanging experience so that you all learn more quickly.