I have a landmass mod i've made some changes to. Some areas i'd like to keep and some I want to delete. Can I save changes made in a single exterior cell to an .esp? Or maybe save interior cells? Then I could take the vanilla landmass mod and load in the new .esp. Any ideas?

You can save the mod as a seperate esp as many times as you like. Use something like Enchanted Editor to remove all the references that you don't want in those seperate esps (ie. the cells that you don't want) and you will be left with an esp with just the cell that you desire. This new esp can be merged with your new mod.

Good to know I don't have to start from scratch. Thanks again.