How to change existing policies in an organisation?
You have to understand that processes change people who follow them. As people learn, internalize and get better at a process, it changes the way they learn how to solve a particular problem. A set of similar processes reinforce each other into a mindset the person uses to solve a category of problems and eventually form a set of values that guides decisions and new solutions to new problems.

Even if you change the process, without the change to the mindset and even more crucial to the values, the person will simply adapt the new process to conform to the same values, same mindset or even same solution as in the original process. At a certain point, it is not possible to divorce this person in this position from the mindset acquired or change the underlying values.

To institute a change, you have the following two options: 


Bring in a person who will already have the right values and mindset and in best case scenario, understand the process that needs to be followed without your help.
Bring in and empower new employee, either recently hired, a new hire or a transfer from a different team in the organization and train him in the new process, hoping to instill the new mindset, hoping the new set of values will emerge.


If the change is local, you might prefer an internal transfer as that person would already share the global company wide values which you want to retain. In case of a larger change, you need to bring in someone from outside to have a fresh perspective and to not share the company wide values which you might be trying to change.

The important part is to empower the person, team or business unit to follow the processes and insulate them from the old team, other teams or the rest of the company, respectively, which might still follow the old set of processes. As it is very hard to insulate such change agent from management above, if the change is to be larger, it often needs to follow all the way up the management chain or come all the way from the top of it.

Note: It is hard to bring change to more than just your team without the support of management. Even inside your team it is difficult if others are already set in their ways. For a new team in a new company a successful evangelist can often affect the forming policies even without the support of management simply by being a leader or creating the path of least resistance for others to follow. But in established company, see above.