Well, it just depends on what exactly you have now and what exactly it's going to take to satisfy you. Is there any reason why you couldn't ride it now just as it is?

You can almost never change just one part on a bicycle. Everything is designed to work together so when you change one thing, handlebars for example, you usually have to change something else, brake levers in this case, to get it to work. For every change that you make, you may find yourself buying something else to get it to work. Now keep in mind that if you are paying retail price for all of the parts you are changing, you are rapidly going to catch up with what manufacturers pay for parts so the cost difference is likely to be less than you'd think. When you get done, you'll still have an old bike with old bike maintenance issues only with a few new parts.

A new bike, on the other, will have every single part brand new. Every single part will have been designed to work with every other part. A new bike will also come with a new bike warranty.

A new