CyanogenMod 12.1 nightlies are rolling out for a few days now and it may take a while for most devices to get official support. Either way, remember this is nightly builds we are talking about, which means there could be lots of bugs on the horizon.
CM12.1 is based on Android 5.1 and it will install on top of CM 12. Any and all third-party add-on ZIPs will have to be updated manually if you find versions compatible to Android 5.1.
The CyanogenMod 12.1 build is already available for devices such as Moto G, OnePlus One, 2013 LTE Nexus 7, and Moto E. These nightlies will keep coming until most devices are cleared out.
A milestone CM build will show up soon, and that will make things more stable, if you wish to wait for one.