Windows 2012 CALS Question

We have a domain with all 2003 Standard Windows Servers. We have licenses to upgrade two servers to 2008 R2 Server but we have not yet bought the necessary CALS. Can I buy 2012 CALS and use them for the 2008 R2 Servers? (Bypass buying 2008 CALS and then having to buy 2012 CALS when we upgrade to 2012 Server).