Regardless of whether you feel that you need huge changes or some slight "tweaking", the same principles apply, to arrive at your destination. Whether you believe it or not, your approach will be the same if it is a huge change or if it is a minor one.
I could teach you every chord there is in 3 hours on a piano, to where you could play any that I would call out to you, or read off a lead sheet. This time frame will be more, if you cannot focus well or cannot assimilate new information well. Your intelligence level also will come into play. Still, you cannot absorb a huge body of data instantly. It must be revealed piece by piece.
The same is true with singing. Corrections and the practice involved are incremental and there is an added factor of emotions, thoughts, beliefs and misinformation (of which you are currently unaware) that can slow or stop your progress. You kind of have to clean house before you can totally move in and live there.
If you were flying from the United States to Europe, there would be hundreds, if not thousands, of course corrections along the way. You wouldn't notice them, if you had a competent and considerate pilot. You would notice if he jerked the plane around, forcing it to stay on course, despite wind or other traffic in the air. That wouldn't be a very comfortable ride, now, would it? This begs the questioning of a worn out truism: "You've got to get out of your comfort zone." Really? Maybe you just need to make small changes over a few days, weeks, or months and you can arrive at your destination without bumps, bruises, or mental trauma.