I know that methods and classes mutate and their functionalities change over time. And the day it 'strikes you :)' that it has become gnarly it is time to refac

I know that methods and classes mutate and their functionalities change over time. And the day it 'strikes you :)' that it has become gnarly it is time to refactor. How you proceed to do that is a different issue. Personally I haven't come across a set of guidelines having more in-depth solution than what I have tried to mentioned here which works in all cases. It is a question quite dependent on the type of project you are working on. I think the best person who can properly direct you is the Architect or somebody with experience who knows the project thoroughly.