Using the handy dandy RegShot utility once again...I can see what change is being made when the setting is changed.

$null = WriteValue("HKLM\SOFTWARE\Microsoft\MSSQLServer\Client\ConnectTo","<sql svr name>","DBMSSOCN,<sql svr name>",REG_SZ)

should change the setting to TCP/IP for connections going to whatever server you specicy in the key. For example if the ODBC was pointing to a SQL server name TESTSQL1 then you would do ...

$null = WriteValue("HKLM\SOFTWARE\Microsoft\MSSQLServer\Client\ConnectTo","TESTSQL1","DBMSSOCN,TESTSQL1",REG_SZ)

Give that a try