Ok I've done that, and tried to make a simple change; switching the daedroth and the minator trophies and moving the front hall trophy and stand backwards. I saved that to a new esp and then checked the details in the Data window, and saw that for some reason it showed changes to dialogue and then instead of hitting cancel to exit the data window I hit ok, the CS reloaded everything and suddenly I got a bunch of warnings saying that the great hall cell had been duplicated and stuff. WTF is that all about?
Edit: I tried loading the esp into one of my saves and visited battlehorn but nothing had changed.
OK, here's what I'd do.
1st - make a backup of the original unedited esp in case it gets messed up.
2nd - since you're already working with an esp file, you cannot save it as a new esp I don't believe. (I could be wrong but I think you do that when you are making a new plugin (esp) from a master (esm) file. So just make the changes you want and simply hit save. You should then see your changes in game. However, It could be me, but it seems that Oblivion has a period of time before a cell resets or something so many of my edits did not show up for me immediately. So, I just made a save outside the interior cell at the front door BEFORE I ever entered that cell, and just loaded that up to view my changes.
Hope that helps!
Oh yeah, it's always a good idea to make multiple backups of your progress very often just in case you have a problem. You might just prevent yourself from losing hours of valuable work that way. (I know I did, lol!)