Hi Wayne,
This restriction does not apply to CCM 5.1.2b (as it does to CCM 5.1.3) :)
Release Notes for Cisco Unified CallManager Release 5.1(2b)
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/rel_notes/5_1_2/512bcmrn_1.html
Here is the CCM 5.1.3 note;
Do Not Upgrade from Cisco Unified CallManager 5.1(3) to Cisco Unified Communications Manager 6.x
It is not possible to successfully upgrade from Cisco Unified CallManager Release 5.1(3) to Cisco Unified Communications Manager Release 6.x.
Release Notes for Cisco Unified CallManager Release 5.1(3)
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/rel_notes/5_1_3/513cmrn.html#wp188112
Hope this helps and Happy Holidays!
Rob