Why will was replaced by shall in this sentence?
Solution 1:
This is answered in response to another question. (See, in particular, the second answer.)
"He shall" expresses more certainty than "he will." This is also true for the second person ("you shall" expresses more certainty or determination than "you will"), but is reversed for the first person -- "I will" expresses more certainty (or sometimes determination) than "I shall."