SQL Server 2008 R2 - Upgrade Default DB Instance (Express) to Developer Edition
I don't think it is possible to upgrade SQL Server Express to another edition.  You should be able to use all the features of SQL Server you mentioned (agent and profiler) with the new, named instance (DEVEDITION) along side of Express Edition.

I don't think its possible to rename a SQL Server instance name, at least not with a bunch of headache.

What I would do is:


Detach the dbs connected to your SQL Server Express instance and uninstall SQL Server Express.  Then re-install the dev edition to the default instance and reattach the dbs.