Old post, but new day...

We have an existing named instance of SQL Express. We want to replace it with a full named SQL instance in such a way that it does not affect the application side of the server. Our thought was to install a second named instance, copy the databases from the Express version to the full version, remove (or rename)the Old Express named instance, then use sp_dropserver and sp_addserver process to change the full version SQL named instance to the old express named instance, Stop and restart services. Would that work?

The point may be moot as we are probably going to try it and see if it does work. This is with SQL 2012, by the way.