Here's my two cents.
1. I wouldn't write "hi xyz" as a salutation to my boss. I prefer to write "Dear Xyz (Given name)" or "Dear Mr/Ms. Smith".
2. If I were you, I would like to explain why I have to change the dates when he has granted you a leave. A reasonable excuse would have your boss's understanding.
3. What dates are 1-3? I would like to wirte the form as Nov. 1st to 3.
4. You will be unavailable. Use "future tense" but not "past future tense".
5. Do you think you should just apologise for the change of dates? He might will have an important assignment for you. I would say "I am very sorry for the change and your inconvenience."
Dear Xyz,
I am writing to tell you that I will be unavailable from Dec.1 to 3 but not Dec. 5 to 7 the dates that you have agreed on. I have to change it because.... Please grant my request about this.
I am very sorry for the change and your inconvenience.
Best regards,
Your name (Capitise the first letter)