Application Development Software

I am working with a custom app which is using the versioning, checkin and checkout capabilities.

The documents within this app subclass Document.

The process is as follows:

1. Check out document.
2. Save it to my local machine and make changes.
3. In the checkin process I prompt for a file upload, get the file content, and do an IfsFileSystem.updateDocument() with the updated content. Then I check in.

The problem is that the class type changes from MySubClassDocument to Document.

How do I prevent this from happening?

Thanks, Jim