Could you specify the reason that you want it to point to a different class?

Yes, I am migrating over a site from version 6 R2, which has its own StartPage type that I want to keep.

In version 7.5, I started with the Alloy sample, and am swapping out components (like PageTypes) with ones from my site.

Oh sorry I thought you wanted to swap class on the fly.

The key mapping between the Page Type in the EPiServer database and your classes representing the Page Type Model is a Guid.

If you add the Class Attribute "ContentType" and specify the Guid of the existing page type, EPiServer will connect these two.

The Guid can be found in the database or if you enter the EPiServer Admin mode, find you Page Type and click the Settings button.

In the Advanced area you can see the Guid to your Page Type.

For example

[ContentType(DisplayName = "My Page Type",
GUID = "3e04ba6f-a598-492c-93b9-dcd2de8dd63b")]

Note that you can't have two Page Type classes having the same Guid in the ContentType attribute.