By learning to play in all (or at least most) keys in first position, your improvised lines (i.e., melodies) will be more melodic. By staying in one basic position while transitioning through a series of chord or key changes, the distance between the notes or intervals will be smaller, resulting in what could be called melodic voice leading. Also, you are less likely to go directly to the root note and more likely to use the third or fifth, resulting in a more colorful melody. Once you can play through a variety of chord changes in first position, feel free to explore the rest of the fingerboard. Just remember that one of the improvisor’s main skills is the creation of melodies. Playing the Changes: Major Triads includes a 30 page PDF booklet, 17 videos, and practice tracks. All of the content can be streamed or downloaded.
Keep it melodic.