There has been a very active thread involving the duties of an administrator and what appropriate actions are. The post that was just made to the Board Admin News clarifies Ancestry's position regarding administrators changing subject lines.
Here is that section of the post:
There are also functions that you have access to do but that are not appropriate for an administrator to perform. For example, editing the subject or the body of the message - this is not appropriate as they should remain as the individual posting the message entered them. If a poster has not entered a detailed subject line, you may choose to reply to the message or send them a message and include information regarding how they can change the subject line. You may refer to the Board FAQ for more information - http://boards.ancestry.com/boardfaq.aspx#greatpost