Today, I spent sometime to try converting a SharePoint site collection to SharePoint 2010 Server.
here is a good article on how you can follow and some commands you need.
First, I back up my Sharepoint Content database and restore it to my SharePoint 2010 DB server. Then in SharePoint 2010, create a new Web Application.
Test-SPContentDatabase –name WSSContent_SP2010Migrate –WebApplication http://chidev51:15408/
stsadm -o addcontentdb -url http://chidev51:15408/ -databasename WSSContent_SP2010Migrate
Go to the site setting of new site collection (/_layouts/settings.aspx), Click “Visual Upgrade” under “Site Collection Administration” -> Click upgrade link.
If your 2007 site use some web parts which not installed on the SharePoint 2010, you will meet the problem to open the default page. You can use SPD to create a new default page and right click to set as home page.