Upgrade from sbs2k3 SP2 to SBS2k3 R2

I have a SBS2k3 SP2 runnig in a client of my and i recently added a new w2k3 STD for backup purpose. I had to run adpred /forestprep to update AD schema set from version 30 to 31. My question is am i incurring in a license problem doing that? Can sbs2k3 (Premium) server have a child DC server w2k3 std? The new w2k3 std server had no issue joining sbs2k3 domain
SBS2k3 SP2 still hold all the domain roles. (FSMO)
Thank you all for you time.