Hi @Steven_Chan,
Welcome to the community!
Regarding your question in the title:
what happens if if you miss the “Cut Over” window?
As per the Live Migrate documentation, If the 72 hour period passes, Atlas stops synchronizing with the source cluster. This would be the same case if you had extended the timer and let the new time expire also.
Should I be worried about anything?
The sync destination would not have any changes made to it from the sync source from the time the cut over timer expired.
Hope this answers your questions.
Kind Regards,
Jason