SQL Server 2014 change license key
I have been going through this exercise for a large amount of servers. I am not going to explain why I am doing this due privacy reason.

I will give you high level steps and let me know if this is not enough.


Get binaries for the version you want to change to.
Start setup.ext
Chose maintenance-->Edition upgrade
Follow standard installation process
If you have more than one instnace on a single host choose the
intance you want to change key.
For me it normally takes about 5 minutes.
Will not ask for a recycle of SQL Server engine in most cases.Microsoft engineers recommend you do a recycle anyway.  You can stage the change before you schedule maintenance window to avoid an extra downtime.