Make sure you follow install/upgrade guide and release notes to plan your migration as that will be 90% of the success. Execution once you have good plan together should be straight forward, but there is lot of items to account for hence the importance of planning.
Some things to consider (not a full list) as you are moving to new hardware, will you be doing PCD migration or moving the virtual machines over and then upgrading, in which case you need to consider ESXI compatibility. If you are doing PCD migration will you be changing IP/hostnames if so you'll need to exchange Phone-SAST certs between the clusters, prepare all integration changes (i.e. GW configs). Make sure your licensing is covered as 12.5 will introduce smart licensing. Besides CUCM what other UC applications are integrated, and what is the plan for those, most will probably have to be upgraded as well, some may be doable independently others will need to be done at the same time. Let us know if you have any additional more specific questions and good luck.
