For 7.5, if usage of registry is a requirement, then you can change the following entries:
[HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\Altiris Agent\Servers]
@="<server-name>"
[HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\Altiris Agent\Servers\<server-name>]
"Web"="http://<server-name>/Altiris"
The Agent restart will be required, thought.
Also you can execute the script with GPO and use utility approach described by "cheldt" above. The "AeXAgentUtil.exe /Server:<server-name>" will be enough.