What would happen to my body if I only train my upper body, and instead of leg day, I sprint?
How you react to any reasonably consistent training strategy will depend on a combination of age, lifestyle, genetic factors.
With that in mind, depending on your Genetics, if you decide to use ”Leg Day” for sprinting instead of squats, you’ll probably get better at sprinting.
What will happen to your body? Well, that depends on the factors cited above and how efficiently you train your upper body. But it’s unlikely, unless you started out with pretty tiny legs in the first place, that your lower body will gain any appreciable size or strength.
If size and strength are your overarching goals, the sort of training scheme that you are suggesting here is highly unlikely to yield satisfying results.
Using progressive resistance training strategies on your lower body will, without fail, contribute to overall strength and size.
Substituting sprints on leg day ain’t gonna.