Bar change...from Road to TT/aero setup?

I would just add bars on that is what I did. that way the bike is versatile to road rides and tri races.  tri set up from what i have heard limits you to training and races,   regular drops with a tri bar extenstions you can still do long rides, tri races and group rides. if you a newbie. I wouldn't do a full out swap until you have your mind set. by that time you might just want to bike a full out tri bike. but i'm a newbie also. so maybe my advise it wrong. we will see what other says.  

for me I just added some bars to my normal bike set up.

I would get a set of "shorty" (ITU style) clip ons and put them on your existing drop bars.

There are a magnitude of reasons for this, but mainly... it's going to make life easier for you in the long run. Having to buy new shifters and brake levers, along with making your bike into a sort of "frankenbike" isn't going to be a lot more cost efficient than just buying a new tri specific bike off the classifieds.

Just my opinion though... it can certainly be done if you really want to.

^^^ Chris is wise... listen to him.

Listen to all above.  Just get some aero bars to clip on.  I did it that way for 40+ races.  I've now got myself a TT bike.

I had a friend who used to swap the front end on his road bike to TT in summer then back in winter.  Which is fine, but it took him time, and by the time you buy a new front end, leavers, cables etc. you could get a classified TT for not much more as Chris mentioned.

Also the only thing it is really going to do is save a bit of weight and be slightly more aero by being flatter and not having drops.  Your best aero advantage (I've heard) is the helmet.  And your best weight saving is in the things that move, wheels, crank etc.  Oh and then there is you.  For each additional 1 kg you weigh you have to produce another 5 watts to match it (Tour De France fact).

Get the clip ons!

I've seen this bit of internet lore thrown around a lot over the last couple years, after the article came out that said the helmet is the best aero buy you can make because it the highest time savings PER DOLLAR spent. That's a whole different thing than saying it's the best aero advantage.