[Alex80]

Hi everyone! can you tell me what meaning the word 'wouldn't' could carry if we used it instead of ' didn't' in the following sentence:
* She'll be working late, so I won't call her at the crack of dawn.
The answer(Future in the past) :
I knew she'd be working late, so I didn't call her at the crack of dawn.
Note: It's just an exercise sentence about changing it into the future in the past. There's no context.
As I know , ' won't' should be changed into 'wouldn't'. Why is 'didn't' used instead?
I need an explanation, please!
Thanks in advance