1.  Update TNSNames.ora to include the new machine name. 
The default location of this file is \oraclexe\app\oracle\product\10.2.0\server\NETWORK\ADMIN
For Example:

XE =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = NewMachineName)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = XE)
)
)
2.  Update the Listener.ora to include the new machine name. 
The default location of this file is \oraclexe\app\oracle\product\10.2.0\server\NETWORK\ADMIN
For Example:

LISTENER =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
(ADDRESS = (PROTOCOL = TCP)(HOST = NewMachineName)(PORT = 1521))
)
)
3.  Reboot the machine.
There should be no need to reconfigure the Primavera P6 Project Management alias connecting to the database when using Oracle XE.