For channels other than VL (FPP, OEM, …), there’s no way to downgrade the SBS 2011 CALs. In this case, you can use the activation keys that were distributed by Microsoft to perform an online activation:

20 CAL keys

FGP6P-W4WXW-3T8BR-TC3QG-HQRQG

KJQBH-XJ6YQ-THYJY-PVDCP-87QT6

Q6D3G-M22TM-TQKM4-C7VKX-RBDT6

5 CAL keys

P9QYQ-YKC4G-B3YD6-MM9BT-327FQ

D2WC6-MCQ88-3CTTW-74YTB-XVFM3

RCFYC-X69X4-6FKDT-PWMM7-V33RQ