Here comes the important part.

First we must lock the offending normals, before they can be unlocked. Bit of a mindbender I know, but this will kill them once and for all.

So, Select the problematic faces, Set the Vertex Normal to lock normals and hit apply.
Re-select the offending faces if the selection got lost, and now do the same step with the Unlock Normals optionbox checked, and hit apply.

Now you can invert and manipulate the normals to your hearts content and the problem should hopefully be but an agonizing memory.

That's it really. Hope it helps, so you can move on to the more fun stuff