The nova-compute service is periodically updating hardware (VCPU, RAM, DISK) statistics for a host and is using the host name (check with hostname -f in Linux) to update the database with the available resources.
In cases where the host name has been changed while instances are running, all existing instances still reference the old host name inside the node column of the nova.instances table. All those entries need to be updated in order to get the correct amount of available resources for nova inside the nova MySQL database:
UPDATE nova.instances SET node = '<new host name>' WHERE node = '<old host name>';
Other columns as host, launched_on should be included in a subsequent SQL.