Q:

converting a nine speed hybrid disc to a seven speed drop bar, is it possible?

I have a used Jamis Coda Elite that I'd love to convert to a drop bar bike, but on the cheap.  I also have an older drop bar mountain bike that already has the 7 speed brifters on it.
Can I just swap out the rear cassette on the Jamis for the cassette on the MTB?  Will the rim on the older MTB have room for the disc swap?
The front tires would stay the same; just new cable to the brake for the brifters. Any help would be greatly appreciated.

A:

The Coda's freehub body can take a 7-speed cassette with the addition of a 4.5mm conversion spacer to take up the extra room, which is a common bike shop item.
There are two stumbling blocks though.
First, if the Coda's current left shifter and front derailer come from a Shimano mountain group, which they probably do, then they won't work correctly with STI because the actuation ratio is different between mountain and road. It can be made to work badly but will never be right. So if this is what you have (as opposed to a flat bar road shifter and a road derailer), you'll need a road derailer, and then there's the question of getting one that either plays nice with the existing crank or getting a crank to match it. If you don't want to have to mess with changing cranks, you may look at the IRD Alpina FD, because it's just about the only one out there that has a cage profiled for mountain/hybrid/trekking/etc triples and takes road cable pull.
Second, you probably have normal v-brake-lever-compatible brake calipers, whereas your STIs have road/sidepull/canti cable pull. They don't work together, at all. So you need either road calipers or a cable pull conversion device such as Travel Agents.

A:

Is the MTB a 26" wheel?  The road bike will likely be a 700c wheel, so different sizes and not swappable.
Even if they were the same rolling size of wheel, then brakes are a sticking point.  Either both need to be rim brakes or both disk brakes.  A road bike with disk brakes would still be fairly new and worth more as-is, not chopped around.
You'd have to take the whole front fork from the MTB to the road bike to get the disk mounts, at which point you've made a lightweight frame front-suspension MTB.
Brake cable pull will be an issue too - your brifters will pull less wire than the MTB brakes.  So rim brakes will move less when pulled by brifters.  This means poorer braking and more brake rub because of smaller tolerances.  Any fixers tend to be patch solutions that don't address the underlying causes.
The general consensus is that a drop bar swap is not worth doing UNLESS you have all the parts, and don't mind losing a working bike.

Alternative Consider selling on both of your working bikes, and buy the new-or-used bike you want to own.

