it would be nice to be able to go straight from a color conversion to a b/w one without un-converting and re-converting.
It isn’t really possible, because Negative Lab Pro doesn’t have direct access to pixel data, except at the time of conversion.
However, if you develop as a color film shot, then create a positive copy, you can change that positive copy back and forth between b+w and color using Lightroom’s regular tools. This method will also have the added advantage of letting you adjust the Color Mix settings in Lightroom, which can be really useful!
-Nate