I have had a single node of a pool using a different license server. Temporary, unfortunately. It has expired, and as the purchase process was somewhat prolonged, I have had to extend it. I did not want to disconnect my other-four-hosts-pool from the permanent license server, which works so fine for the last year or so, so I have had to change the license only for a single host.
XenServer 6.1 XenCenter does not allow changing the license server for a single host in a licensed pool. I have had to search for a solution. The solution looks like this:
As for a while now, Citrix do not use license file, but license server, and alternate solution had to be found.
xe host-apply-edition edition=advanced|enterprise|platinum|enterprise-xd license-server-address=<license_server_address> host-uuid=<uuid_of_host> license-server-port=<license_server_port>
This has solved the problem immediately.