Terra Nova: 100% Provided you back up your plugin as an .esp, Creation Kit will not directly edit an ESM, so ticking the flag locks you out unless you use WryeSmash or another third party tool to ESP'ify.
Note: To ESM'ify with CK, change a copied (leave original as backup) file extension to .ESM, open, set as active, save. You will not, after saving, be able to edit anything, but you can upload to the Workshop at that time. Currently, however, subscribers will end up with a false flag ESP, that is, a plugin with an ESP file extension yet with its internal ESM flag ticked. Such plugins work just like normal ESPs, but will always load before a normal ESP.
Mahou: Updates initial reply
