By the nature of the beast, any time you import or update a schema, InfoPath will lock it because it cannot guarantee where it came from. Only schemas built directly in InfoPath (yeah, yeah, you could argue that it was -- but that was from a different form) are editable.
Now-- one thing you can do, is extract the form files from both forms (the one with the editable schema, and the one without), and then compare the differences in the manifest.xsf file for each. I don't recall off hand what they are, but there are a small number of key differences that cause the lock.
If you attempt to do this with a schema that uses a namespace other than "my:", then weird things can happen. This is not recommended.
In fact, this is an advanced process, not for the light-hearted. Be sure to back up your form before trying this.