(0) Learn how they do things there, and why, and gain the respect of your coworkers by getting really good at it. You won't find you can make much progress along any axis without people perceiving you as already being successful.
(1) Write all the unit tests you possibly can for as much of the codebase as you possibly can, and try and get the rest of the team onboard with a solid testing philosophy. Extoll designing for testing and testability.
(2) Use these to confidently move through the codebase and replace portions as necessary.
This is the 'replacing the engine while the car is on the freeway' approach.
(0) Learn how they do things there, and why, and gain the respect of your coworkers by getting really good at it. You won't find you can make much progress along any axis without people perceiving you as already being successful.
(1) Write all the unit tests you possibly can for as much of the codebase as you possibly can, and try and get the rest of the team onboard with a solid testing philosophy. Extoll designing for testing and testability.
(2) Use these to confidently move through the codebase and replace portions as necessary.
This is the 'replacing the engine while the car is on the freeway' approach.