You can usually convert any evaluation version of Windows Server 2012 to a full version by entering your product key using a special command; however, based on the linked article, it seems you can't do this on a domain controller, so you'll have to install a new one.
The easiest way to move AD to the new server is to simply make it an additional domain controller in your already-existing domain (making sure to also enable the DNS service and the Global Catalog role), and them demote the old server; this will replicate AD to the new server and it will also move all single-server roles to it. After this is done, you can simply shut down the old server and throw it away.
If you find again in need to install a temporary server which may or may not be converted to a production server in the future, just use a standard installation DVD (or ISO) and don't enter any product key; you'll be free to use it for 120 days, after which it will start prompting you for activation (but it won't stop working, it will just remind you everytime you log on). This is exactly the same as an evaluation version, but you can subsequently activate it by simply entering an appropriate product key, without the need to rebuild anything.