Hi Paul,
It can be an upgrade as well as a migration depending upon whether you are trying to change the hardware. Please check the following link
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/ucmap/10_0_1/CUCM_BK_U9C58CB1_00_ucmap-administration-guide/CUCM_BK_U9C58CB1_00_ucmap-administration-guide_chapter_011.html#CUCM_TP_SFE27465_00
Migration to 10.0 Cluster
- Unified CM Releases Supported: 6.1(5), 7.1(3), 7.1(5), 8.0(1-3), 8.5(1), 8.6(1-2), 9.0.(1), 9.1(1), 9.1(2)
- Cisco Unified Presence (CUP) Releases Supported: 8.5(4), 8.6(3), 8.6(4), 8.6(5)
- Cisco Unified CM - IM and Presence Releases Supported: 9.0(1), 9.1(1)
- Contact Center Releases Supported: Migration Not Supported
- Unity Connection Releases Supported: Migration Not Supported
Upgrade Task - (Upgrade Application Server or Install COP Files)
- Unified CM Releases Supported: 8.6(1-2), 9.0.(1), 9.1(1), 9.1(2), 10.0(1)
- Cisco Unified Presence (CUP) Releases Supported: 8.6(3), 8.6(4), 8.6(5)
- Cisco Unified CM - IM and Presence Releases Supported: 9.0(1), 9.1(1), 10.0(1)
- Contact Center Releases Supported: 9.0(2), 10.x
- Unity Connection Releases Supported: 8.6(1-2), 9.x, 10.x
HTH
Manish