I have my esp mod file done and published, being used by subscribers, etc. Now I want to make an optional esp file with Dawnguard additions for people with the DLC, that will overwrite my original mod. I tried just doing it and saving as a new file, but the CK forced all my changes into a new duplicate cell. What should I do to make it work?

Copy your esp to another folder. Rename the esp in the Data folder with a _DG suffix. Load the YourMod_DG.esp into the CK and make your changes/additions with the Dawnguard assets. Save your esp. Copy the original esp back into the Data folder.

Now you'll have 2 versions of your mod -

MyMod.esp (normal vanilla Skyrim) and
MyMod_DG.esp (the Dawnguard variation)

Only ever have one of these active in the CK and in the game. You can realese both of them in seperate packages and make users aware that they can download/use one or the other ingame,...not both.

Yeah, that was the backup plan. But it's a large mod, so I was hoping to avoid that for upload reasons.

I've read that there are ways to script the Dawnguard assets, so that they only show up if the Dawnguard files are present. Someone with a lot more scripting knowlege than myself, would have to give you instructions.

http://www.creationkit.com/GetFormFromFile_-_Game#Examples, I'd imagine?

http://www.gamesas.com/user/791299-bw117/: If you need to edit cells, you're probably better off adding Dawnguard as a secondary master. The engine has no problems with ESP masters, but the CK won't allow it. Make a copy of your ESP, open it in WryeSmash, R-Click and copy to ESM. Then open up your ESM and Dawnguard simultaneously with no active file, saving the edits as your Dawnguard supplement. Once it's completed, edit the masterlist of the result with WryeSmash to look for the ESP version rather than the ESM and all will work in game. If you need to edit the supplement with the CK, you'll have to repoint its masterlist the the ESM version and back again.

I've already just made a copied version of the mod with added Dawnguard assets as a whole new file. Thanks for the help though, I just don't have time to do much coding or esm-ifying at the moment.