To be transformed from one form into another by a process of gradual modification, esp. from a more rudimentary to a more highly organized condition - Oxford English Dictionary
The G-7 member countries are saying that their current policies in relation to their central banks lack the depth and maturity that is needed for the job and that improving them will be a process of many steps - the article gives examples of the things that need to be done.
So, rather than a single, radical change that solves everything they are proposing that the policies grow in the same way that an organism evolves to meet the challenges and opportunities of its environment