Hi Louis,

The direct upgrade path would be CUCM 5.0 > 5.1(3) > 7.1(3a) as shown here;

Cisco Unified Communications Manager Software Compatibility Matrix

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/compat/ccmcompmatr.html#wp1569721

Here are some other excellent references;

System Installation and Upgrade Manual for IP Telephony Enterprise and Midmarket

http://www.cisco.com/en/US/docs/voice_ip_comm/uc_system/UC7.0.1/ipt_system_inst_upg/suimt701.pdf

Cisco Unified Communications System Documentation

http://www.cisco.com/en/US/docs/voice_ip_comm/uc_system/unified/communications/system/ucstart.htm#Cisco_Unified_Communications_System

Cisco Unified Communications Compatibility Tool

http://tools.cisco.com/ITDIT/vtgsca/VTGServlet

Hope this helps! And best of luck!

Rob