I have more or less modified my 4500 into a ATB (All Terrain) or urban cruiser. The first thing that goes are the knobby tires, in fact when buying a new bike swapping them with something else has come to be part of the deal. My tire of choice for bike paths and commuting is the Michelin City Tire or CST Selecta in 1.8-1.9 inches - wider is too much rolling resistance. These have no knobs, but some negative tread to handle water. Tires with a smooth center and knobs on the sides work for crushed rock etc but I do not like they way they handle on corners on paved surfaces. Smoother, narrower tires make a very big difference.
The next thing is a fork that can be locked out. Especially at lights, on hills or anywhere you want/need to accelerate the fork can absorb too much energy. Mine is variable so I run it at about 85% lock out so it can handle the odd pothole etc.
But that doesnt make it a hybrid IMO. I have a pair of 7.5FXs with wider than average tires (38s) which are much, much easier to ride and ride fast mainly because it is much lighter than a MTB frame. I dont think they are quite as comfortable to ride owing to the narrower tires.
The ATB/urban cruiser is harder to ride, but I want to burn calories/get a work out when I ride, so it doesnt bother me. My average non winter ride is 40 miles, so to do the same work on the FX I have to go much farther or faster and that would cut into playing poker time
