I guess we've all had this problem when trying to discover how Bethesda implements something.
Is there any way of loading a cell in read-only mode, without unloading the plug-in?
Just do what I do. Have a second copy of the CK open to use to poke around with stuff to see how it was done or check on things you need.
So, uh... how do I make Update.esm a master if I've already got the mod up and running? Do I just have to load the esm together now and save the plugin again?
Edit:
Also, if, say, update.esm changes the record of Iron Armor from Skyrim.esm, when I load both of them, which record will appear? From SKyrim? From Update? Or both?
Just make sure Update.esm is checked to load before you select your mod to load. The CK will take care of adding it.
When Skyrim.esm and Update.esm load, anything Update.esm changes will be what you see while working.