You could uninstall the agent on these computers, by doing an Uninstall Push from your new, functional NS server. Then rediscover or repush the agent with the correct server name.
If the trial server is at all functional for any purpose, even with an expired license you should be able to change the targeted agent settings policies to use an Alternate URL for the agent, thus redirecting it to the new NS:
http://www.symantec.com/docs/TECH127618
You could use RAADv2 to connect to each computer and provide a new agent URL:
http://www.symantec.com/docs/HOWTO21449
See the 'Redirect agent' button in the RAADv2 screenshot within that KB.
All of these assume you cannot manage the computers using a previous tool, a logonscript, etc -- only using Altiris.
Do any of these methods work for you?