Dave
USA My initial reaction is no, you cannot. The direct vent model is manufactured and tested the way it is. The venting is made specifically for that heater, that specific combustion air flow, that pilot flame, that ignition, and of course, that venting etc... The best way to find out is to ask the manufacturer if it can be done, and what items need to be changed / modified. If the mfr says yes, then make sure you follow their instructions to the letter, or you risk fire and/or carbon monoxide poisoning. I do not know what you mean by a "regular" vent, so I will assume you mean non power-assisted venting. I do not know what water heater you have, where in the building it is located, and what type construction you have. Therefore, the best place to get information on gas vent requirements is to look up the different vent types in the 2006 International Fuel Gas Code, and find the type of appliance you have. The Int'l Fuel Gas Code also covers commercial installations. The sections applicable to single family homes are included in the 2006 International Residential Code. So, be safe and get a copy of the fuel gas code or residential code for the latest and greatest facts and code requirements.

rangedog
Your direct vent water heater draws it's combustion air from the outside atmosphere. If you go to a regular water heater, you may be able to use the same building penetration for the flue, but chances are, you'll pull combustion air from the attic space through one or two 4" duct or single wall pipe. Shouldn't be hard to go to a regular heater, but direct vent water heaters are still available. Pricey but still around.