I would suggest that you contact your certification body and explain the situation. They should be able to provide guidance to your question. In the end, you are their customer and they should try to help you.
From what I read in BRC version 7, section 2.7.2, you can postpone the audit in case you are producing seasonal products and the production is delayed by a late start to the season without getting a non-confirmity for point 1.1.8. But your certificate is only valid until the date written on the certificate. After the day of your audit, you still need to wait 42 days until your new certificate will be issued. By then your customers will find out that you don't have a valid certificate anymore. You can always explain them the situation, provide evidence of the reason for delay, a report of the audit non-conformities and hope that they will be reasonable and understand it.
If you are going to be audited after the 1st of February, it will be done against version 8. I just attended a workshop on BRC version 8 at which a fellow attende was saying that they have unannounced audit in the second part of January - first part of February. The BRC auditor that was also at the course said that if the auditor comes on the 31st of January, then it's version 7, on the 1st of February is already version 8.
Can I ask what kind of product are you producing?