When an organization is undergoing a cultural change, the behaviors that were rewarded yesterday may not align with the behaviors expected tomorrow. The tenured employees who have been loyal and depended upon may not exhibit the values and practices that are needed for the organization to be successful going forward. And if the organization is trying to become more agile, it may find that Generation Y grasps some of the changes more readily. The team members who understand the goals and new way of working may be the "junior" ones... and the ones who struggle to adapt may be the "senior" folks who have contributed a lot of value over the years.
It may be hard, but it's important to reward the right behviors, and that may mean taking a second--and third--look at the people you've grown to count on to coach them appropriately as the culture changes.