Nope, once created, you cannot apply  a new template.

Nope. In WSS v2, once a site  is provisioned, the site definition it uses
is fixed.

Is this something that is available in V3?

Not really. While you can't change the underlying site  definition,
Features can be activated and deactivated at the Farm, Web Application,
Site Collection, and Site level.

So, if the changes that you want are bundled into a Feature, you can
grow existing sites  to have whatever functionality that you would be
changing the site definition to get.

For example, a Blank Site in WSS v2 will only have the latent
functionality that it had when it was created. But in WSS v3, you could
grow a Blank Site into a Publishing Site or to have whatever
functionality you bundled into a custom Feature.

Sounds like a good feature. I would have to learn more about V3. I suppose that it’s too bad that we just developed something for a client using V2 when V3 was right around the corner.

Which product?

You cannot “apply” a new template. The template  is used to create and provision the site  during creation.

However, with MOSS07, you can modify the features activated on a site collection and subsites which could have the impact of applying  a new template.