What happened to Smigrate.exe? What if I only want to upgrade one site? What can I do now with Stsadm.exe?

Question listed on the WSSFAQ

There is no command line utility replacement for smigrate. If you want to upgrade one site at a time you can still accomplish this. Let's look at the different methods and how you could accomplish this...

Re-reading my post I can see it got away from a simple little answer. I hope you don't mind my snipping content from TechNet and MSDN.

The Three Upgrade Methods -

Gradual
You can choose individual sites to be upgrade when running both versions of the product on the same farm/server. This would give you control of choosing which sites are upgraded and when.

Database Attach
Backup and restore a site into it's own database (steps referenced below in TechNet section) then attach the content database into the farm you want it to belong to.

In Place
You could make a copy of your entire environment (could be on a virtual server) then upgrade in place, finally delete all the sites you don't need. Not sure if that is useful. If you're only wanting to upgrade a single site from a V2 farm to a V3 farm the database attach is the easiest.

Migration

Some thought of the smigrate utility as being the tool for migrating sub sites aka webs. You can now do full fidelity (including security) site collection and sub site backups with stsadm. For full usage type from your /bin directory....

stsadm -help export

[stsadm -o export -url URL -f filename]

stsadm -help import

[stsadm -o import -url URL -includesecurity]

(more details below)

Here's an edited snippet from the MSDN documentation:

Ways to Use the Content Migration APIs

There are three ways you can invoke the Content Migration APIs.

1. stsadm.exe

Using the stsadm utility, you can use the import and export operations to migrate data.

2. SOAP

You can use the ExportWeb and ImportWeb methods implemented in the Sites Web service to migrate data from a remote server.

3. Content Migration object model [The Richest if you have dev resources]

The object model provides the most control over your data migration scenarios. Using the object model, you can migrate anything, from a Web site to an item in a list, or a single document in a library. You can choose whether to include information about security, versioning, user roles, and other metadata appropriate to the objects you are migrating. The Content Migration object model is implemented in the Microsoft.SharePoint.Deployment namespace.

Here's a snippet from TechNet...

Migrate content by using import/export

The import/export feature is based on the new Content Migration APIs. With import/export, you can migrate either subsites or entire site collections, and you can import a subsite into an existing site collection. Like the Smigrate.exe utility in the previous version, import/export requires that the site that you import to already exists. Note that import/export does not include some site settings, such as Recycle Bin state and alerts. For more information about what settings are preserved during import/export, see <Link to come>. To use import/export to migrate a site, you would use the following process:

1.	Export the subsite or site collection (stsadm.exe -o export -url URL).
2.	In Central Administration, on the Manage Content Databases page, set all databases except the one that currently contains the subsite or site collection to offline.
3.	Create the site collection to contain the content you are importing (only if importing an entire site collection).
4.	Import the subsite or site collection (stsadm.exe -o import -url URL -includeusersecurity). The includeusersecurity parameter specifies that you want to import the security settings for the subsite or site collection. If you do not need the security settings, you can omit this parameter.

Migrate content by using backup/restore

You could backup a site collection, then delete the site collection, set the manage content databases to take the current database offline [joel: I don't recommend taking them offline, the capacity option works much better without offline dbs] (or set the capacity to the content database you want the site in so that it has more capacity than the other content databases, and then restore the site to original URL and that would get the site collection in the new database. The main thing to note is that this feature is at the site collection level so would not be an option for moving subsites around. This feature can restore a site collection to a new name and the site cannot already exist unless you are using the –overwrite option to restore over an existing site. To use backup/restore to migrate a site collection, you would use the following process:

1.	Back up the subsite or site collection (stsadm.exe -o backup -url URL).
2.	In Central Administration, on the Manage Content Databases page, set the database that currently contains the subsite or site collection to offline.
3.	Restore the site collection (stsadm.exe -o restore -url URL).

For more information about using backup/restore, see Office SharePoint Server 2007 Backup and Restore .

With Office SharePoint Server 2007, you can migrate sites, subsites, or specific lists/libraries or items/documents by using the content deployment capability. This method is as easy as specifying the content to deploy and the target for the deployment, and then starting the deployment process. For more information about content deployment, see the Content Deployment Admin section at RTM.

Recommended Reading:
Gradual Upgrade Detailed Steps blog post by Shane Young

How to migrate a sub site to another location - Content Migration API content on MSDN

Migrate content or sites after upgrade - TechNet