Muscle adaptation is nothing more than the fact that training causes your body to adapt to the training and thereby get stronger, bigger, and better at doing the activity without tiring. Switching up a workout is good in order to avoid hyperspecialization in a specific version of a movement, but there's a misconception that one should switch workout programs entirely just for the sake of somehow "confusing" one's muscles. If you're still progressing with a program there's no need to switch.
Similarly, though fewer reps are better for training strength, the "five reps" recommendation is generally applied to compound barbell movements, where one can carefully control the amount of load applied to the movement. Five isn't some magical number: it's just that strength is defined as the ability to move a load once, not move it many times, so the closer you get to one maximal rep the closer you are to training for pure strength. Five reps is merely a good middle ground between grueling high-intensity maximal efforts and higher-rep work that produces strong tendons, endurance, and other desirable qualities. Don't put a cap on your training at five reps, especially with upper-body bodyweight exercises, which generally require a different approach that allows for greater volume.
Specifically, if you can do 15 or so pull-ups in one set, don't reduce your total pull-up volume while working on one-arm chins. You still need plenty of high-rep work in order to keep your current level of ability.