Cal Packs for SBS2003 Standard

Dear Experts,

I have a server that was installed in Dec 2005 running SBS2003 (standard) OEM.
During the installation it also had 2 x CAL packs totaling the licenses to 15 CAL.

Since 2005 and lots of windows updates later we currently have SP2 installed, but not upgraded to R2.

I'd like to add another 5 CAL pack this system, but the only part codes I can find availble are for SBS2003 R2.

Question 1:  Can I install a 'Small Business Server 2003 R2 User Client Access License' pack on a non R2 server?
Question 2:  I'm not keen to move to SBS2008 but am interested in upgrading the box to SBS2003R2 - Do you know if the R2 upgrade kit is still available?
Question 3.  It appears that SBS2008 5 CAl pack is cheaper then 2003R2 5 Cal pack - Can I use the SBS2008 Cal on a SBS2003 or SBS2003R2?

Thanks for helping me out