I am not sure what that would do to the installer. If the installer tried to open multiple connections it would fail, and it may in fact change the mode itself during the upgrade process. Not sure about 2000 but 2005 and the newer will deny connections during critical portions of an upgrade stating they are in 'administrative mode' or something like that.

Is your goal simply to guarantee users cannot connect to the database instance during the upgrade? If so then if you only have network clients to worry about then consider disabling all protocols except Shared Memory to essentially take the database instance off the network before doing the upgrades.