How Easy/Hard is it to Change a Tri Bike into a Road Bike?

I'm thinking you should just need a set of hex wrenches and time.  Have your road bike handlebars already setup with the brake/shifter levers and have the cables run thru them.  Remove the tri bars with shifters and brakes.  Install road bar assembly.  Route cables and adjust. 

You might also need a different seat position.  Maybe a dual position seatpost to reduce the risk of messing up your fit.

I've never heard of century rides that have rules against tri bikes. I see lots of them in the rides I've done and I rode mine.

The century probably does not allow tri bars, and have nothing against tri bikes per say... if all you want to do is to be able to use regular drops once in a while, then go a head and follow KenD's recommendations, that will work just fine... but if you are actually planning to use the bike as a road bike, then I would say it's near impossible to take a proper tri bike (with tri geometry) and make it a road bike... you can't change things like seat tube angle (steeper on a tri bike), headtube geometry (slacker on a tri bike) and top tube (often shorter on a tri bike).

just something like this...





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That works! Still... I wouldn't ride that as my everyday roadbike... sure it does look cool, but handling and more than anything else, climbing, would be a dog.

Yea just considering doing it to save me only having one bike.

Just so I can use it for races that would be better suited to not having the big tri bars on them.

Do you have STI shifters/brake levers? Like the ones on road bikes? If not, don't bother investing any money in this idea. 105 shifter/brake levers will run you well over $200 dollars. Ultegra start at $330 and Sram is just as costly. Your best bet is to save that money and buy an inexpensive road bike.