If I want to change a flavor I have in the carto part of my carto tank, I a.just add a new flavor that is similar (have found some good combos that way) b. or if it's a strong flavor, unflavored will help that a bit.
The reality is that I usually vape the same juice for the life of a carto, but occasionally I will run out of that juice before the carto dies. And I just add it to the tank, not the carto. It will all blend eventually
Have fun when that happens when you are vaping banana - that's a hard one to find similar flavors to.
ETA: As for actually cleaning the tank, I do so when I change out the carto. Just take it apart and rinse. If it's a strong flavor juice, soak in alcohol (clear) for a few and rinse.