Muscles are always been torn down and rebuilt to maintain steady body-weight. What you can do is tweak those rates in the direction you want. Bodybuilders modify on the building end, you can tweak it on the tear-down side.
Muscle strengthening and growth comes from causing microscopic tears and trauma to the muscles fibres through exercise. This leaks out the contents of the cells into the surrounding tissue. This causes a rebuilding response and accumulates histamines, inflammatory cells, prostaglandins, fibroblasts, etc. The rebuilding process then uses free amino-acids and proteins absorbed from the bloodstream to rebuild these torn tissues.
Your protein consumption doesn't directly cause muscle-building or destruction. You cannot gain 5-7lbs/month of lean-muscle mass by just sitting around and eating steaks. You must workout intensely in the gym like the top bodybuilders to gain that amount of muscle. Similarly you cannot lose muscle mass as well by decreasing protein-intake (vegetarians don't waste away to nothing). Exercise is necessary in both cases to build or lose muscle. To lose the muscle-mass, you must utilize it as fuel during exercise. This requires long endurance rides where you deplete your glycogen stores and force your body into dissassembling muscle-tissue for fuel. Then maintain a calorie-deficit with your diet. However, you cannot control where this tissue is destroyed, most likely all over.
However, you can
control what's rebuilt. Due to the inflammatory rebuilding response, only those tissues that have been stressed, damaged and leaked their contents will be rebuilt. That means muscles that have been exercised and stressed will be rebuilt. So the areas you want to keep strong, you must exercise while not pushing the muscles you want destroyed. So, don't do any upper-body workouts, don't yank on your bars. However, you do want to do hillclimbs and intervals to stress your leg-muscles and heart. Combine with the long endurance rides over time to lose weight in fat and muscle, you'll see a gradual shift in the muscle composition of your body; more muscle in the legs and less in the upper-body.
WARNING: The part I'm unclear about is if it's possible to lose muscle-mass all over while maintaining the same in the areas you want to preserve. It may end up being that you lose -20% of upper-body mass at the same time as -10% of the lower-body muscle. There may be a combination of a calorie-deficit diet and exercise that may allow for a -10% upper-body muscle while keeping all of the lower-body. But I'm not sure of the exact combination. You'll need to track diet, exercise, body-fat % and lean-muscle mass numbers over time to adjust as you go. You will find that with this kind of an exercise programme, you will not be making much improvements in fitness and speed. Many people experience constant fatigue, lack of energy, lethargy and apathy, so be careful.
Some articles of interest:
Journal Applied Physiology - Muscle regeneration during hindlimb unloading results in a reduction in muscle size after reloading
Journal Applied Physiology - Isometric Resistance Exercise Fails to Counteract Skeletal Muscle Atrophy Processes During the Initial Stages of Unloading