Hi David,
One of the most common things that people miss is the need for a new "Software feature License" when going from 6.x to 7.x. If this is not loaded after the upgrade, the CCM service will not start. If you give us your exact From/To versions we may be able to fill in a few more blanks here :) These documents/Links below are sometimes over-looked but conatin significant UC upgrade Paths/strategies/recommendations. There is alot of info to weed through but will certainly save you from any potential "gotchas".
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
