Getting Biometrics Appointment Date Changed

[ghalibm]

Work Start Date: 1st of February
Date Visa Application Submitted (Online - Paid): 26th of January
Biometric Enrollment Appointment: 13th of February
COS Expiry Date: 8th of April

According to the submitted application, I have to submit Biometrics by 30th March.
I am thinking to change my biometrics appointment date from 13th February, to somewhere in the mid March.This way I will have my original transcript to submit, instead of relying on an academic reference.

Questions:
- Is there any limit to the number of days after work start date that biometrics have to be enrolled?
- Is there anything else that I should be concerned about while making this change?
- How long before COS expiration date can you make the application?

Any help will be appreciated.

Thank You,