This is a hard question. The fermented beer is definitely different. Over time you can gain a sense of how it will change. So, yes it will taste different after fermentation, but it won't become less bitter. If anything the reduction of sugars will cause the bitterness to stand out, but that is all relative to other changes in the flavor of the beer through fermentation.
I take it you have made a beer you think is too bitter out of the kettle?