How do I convert a Plugin to a Master file, and what are the repurcussions?

Background: Several fans of my mod are asking to implement its main features as a Library so they can write compatibility mods to make it work better with other mods like Hypothermia and ImprovedFish. This would also allow use by people who don't want the cell/world changes I'm making, but still want the nets, poles, and dynamite craftables.

Am I correct that I need to move these items to a Master file, then? If so, how do I do the conversion? I tried simply renaming my mod from "FishingInSkyrim.esp" to "FishingInSkyrim.esm" but the CK is still listing it as a Plugin, not a master.

Change extension to ESM, open as "Active" in CK, save, done. For others to add it as a master with the CK, it will need to be a *bona fide master.

*both flag and extension