Number of licenses from version 11 to SEP 12

A client needs to renew the following product.

SYMC ENDPOINT PROTECTION 12.1 PER USER RENEWAL ESSENTIAL 12 MONTHS EXPRESS BAND C

They are currently using version 11 and have increased around 20 additiional temporary clients now. Since the license price is expensive, they may not purchase enough licenses at this moment.

If upgrade SEP version from 11 to 12, will it not allow to use or connects sepm 12 server?  Any problems will be happend? Please advise.

Thanks.