Requirements for Transformation
In order to perform this transformation in a relatively short period of time a day, our lycanthrope is going to need a lot of calories. A lot. Consider that active male teenagers are well known for consuming impressive quantities of food on a daily basis while undergoing their growth spurt, a lycanthrope about to undergo a transformation will have similar caloric needs in a vastly compressed time frame.
Calcium & Phosphorus Requirements for Bone Growth
According to this article, the ratio of calcium to phosphorus is 5:3. If between the ages of 10 and 20, the average human male accumulates about 17 pounds of bone to achieve a height gain of 15 inches, that means our lycanthrope needs to consume 0.5 lbs of calcium for each inch of height gained. This quantity doesn't account for any increases that might be required for increased bone strength as a result of the cube-square law. Please note that 0.5 lbs of calcium is just for structural growth. Calcium is also an electrolyte so you'll need extra to counteract losses through sweating or urination. In terms of food consumed to get this much calcium, assuming a dry skeleton is 20% of total body weight, our lycanthrope will need to eat the bones of two chickens for each inch of height he grows.
Speaking of chickens, a 140 gram (5oz) chicken breast offers 43 grams of protein. For each inch a human male grows during puberty, he gains two pounds of muscle. Since muscle is approximately 80% water, each inch of growth requires 181 grams of protein (and 726 grams of water to hydrate the muscle (about a 3/4 liter)). The lycanthrope must eat four chicken breasts to get sufficient protein to grow an inch. In addition to the chicken breasts, he will need to drink half a liter of water just to hydrate the new muscle.
Ha! This is working out nicely, each inch of growth for the lycanthrope equals two chickens for calcium and protein. Better eat the whole chicken though instead of just the breasts because dark meat has lots of fat in it.
Digestion Caloric Intake
Each calorie a human consumes carries a 10% digestion cost so all of our estimates thus far will need to increase by that much.
Brain damage in humans happens when body temperatures go above 107°F (41.6°C) thereby setting an effective limit on metabolic burn rates. Elevated body temperatures may inhibit some metabolic reactions (though I'm not biochemist so I can't say for sure). Increased body temperate means lots of sweating, so he will need even more water and electrolytes just to stay alive.
Thermal Output of Growth
To simplify calculations, let's assume the lycanthrope is just a water bag weighing in at 100 kgs at 35°C. Note that a nutritional calorie is 4.2 kilojoules. To raise the temperature of one gram of water by one degree Celsius takes 4.179 joules. Assuming no heat loss, the lycanthrope has a 2.7 megajoule heat budget going from 35°C to 41.6°C. (I'm going to ignore natural heat loss through radiation or sweating. My physics are not that strong.)
Let's say our lycanthrope is going for 2 inches of growth for this transformation. That's four whole chickens or 6 lbs of which only 30% is protein, fat or bone. We need to digest 1.8 pounds of raw materials. To make calculation easier, I'm going to assume a uniform digestion cost of 0.7 calories (2.94 kjoules) per gram. (Fat yields 9 calories or 37.8 kjoules per gram. Protein yields 4 calories or 16.8 kjoules.) 1.8 pounds is 816 grams. 816grams * 2.94 = 2.39904 megajoules, or just under our heat budget of 2.7 megajoules.
Keep in mind that this thermal output is just for digestion and does not include basal metabolic rates or the cellular work required for transformation. Regrowing bone and muscle is expensive so I would expect increased energy costs to do it.
Eating two chickens plus other foods plus drinking enough water is going to be a severe test of digestion. Buffering the minerals, fats, and proteins for use during transformation will also be difficult as the circulatory system can only hold so much. Reconfiguring muscles and bones in a day or so could be possible but growth that rapid is akin to extremely aggressive forms of cancer. In short, our lycanthrope is going to be a hot, sweaty, greasy, stinky, comatose mess during the transformation. From his perspective, it will feel like every bone is broken and every muscle will have third degree muscle tears. So either he gets really lucky with his transformation where it completes and he's completely healed or it's gonna take a few days/weeks to recover. Also, the lycanthrope can't eat solid food while transforming because his jaws may lack the strength to actually chew.
So yeah, it's possible over a period of weeks but it's gonna be really really really uncomfortable. Transformation in less than a day isn't feasible.