I searched on Google because of grammar about 'reported speech'. The left on below is what typed. I understood I did mistake or error about the word 'will' and it should be 'would', but isn't it correct to change 'tomorrow' into 'the following day'? I learnt 'the following day' and 'the next day' are both unobjectionable when changing sentence structure like this before from a book.
5) Emily:"Our teacher will go to Leipzig tomorrow."
Emily said that her teacher will go to Leipzig the following day..
Correct answer: Emily:"Our teacher will go to Leipzig tomorrow."
Emily said that their teacher would go to Leipzig the next day
Sources from Reported speech - statements with expressions of time - English
There is no difference between "the following day" and "the next day" in this context.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.