The gradual upgrade option as we knew it is gone in SharePoint 2010. Good riddance! As Joel Oleson explains in Is Gradual Upgrade in SharePoint 2010 Really Dead? How is Upgrade Better? visual upgrade is the new gradual upgrade. Visual upgrade gives you full control over the switch to the v4 UI for your sites and saves you from trouble of running two versions side by side.
Okay, this is great when you can complete the database upgrade part over a weekend. If all goes well (you tested it several times, right?), your users might not even notice their sites are now powered by SharePoint 2010 when they get back to work on Monday.
What if your databases are too many/large to upgrade that fast? Microsoft does help to mitigate downtime with SharePoint 2010 upgrade: you can toggle your original databases read-only while you upgrade their copies in parallel on a 2010 box. Still it may not be enough - you may have site collections that need to be writable and accessible most of the time. With a large web application, upgrade may last days (db upgrade + operational overhead). And all this time your v3 web app is read-only at best.
Is there anything you can do about it? In fact you can. Microsoft has extended alternate access mapping (AAM) in SharePoint 2010 to enable upgrade at a more granular level than an entire SharePoint web application.
How it works? You can configure your 2010 web application to redirect incoming requests for site collections not found in the attached databases to a URL of your choice. Since the URL can be of your original SharePoint 2007 web application, you can use AAM redirection to make your database upgrade more granular. Just set up redirection and start upgrading the databases. Initially your new 2010 web application is empty and redirects all requests to 2007. As you attach databases to SharePoint 2010, it will be switching over to the upgraded sites. By attaching your most critical site collections first, you will let your users fully access them well before the rest of the sites are upgraded and writable.
The catch? Using this process requires additional administrative efforts and multiple manual operations. You may also need to work around some issues caused by URL redirection (e.g. fix InfoPath form templates). So you should carefully consider pros and cons before you proceed.
In the next installment of this series I'll walk you through the process of using AAM URL redirection when upgrading your web applications to SharePoint 2010.
AAM redirection white paper on TechNet