One quick suggestion:
Take initial catalog=old_db; out of the Main.ConnectionString value.
Then it will should use the inital database connection for the current user (configured through SQL Mangement Studio.
BTW, just picking nits, but dbo is the schema, not user.
Yyou know this only works for non web apps, right? Web apps use web.config.
And app.config gets copied to debug/release folder as yourappname.config
My App.config has:
<add key="Main.ConnectionString" value="Server=SERVER01;Trusted_Connection=true"/>
<appSettings>
<add key="Main.ConnectionString" value="data source=localhost\sqlexpress;integrated security=SSPI;persist security info=False;packet size=4096"/>
</appSettings>
Just trying to cover a lot of the frequent issues.