I'm thinking of switching to a steel frame road bike and I am toying with the idea of simply buying a frame and moving all the components and wheel-set over from my current road bike as an alternative to buying a new complete bike. Through work I can get a deal on Soma frames and I like the looks of the Soma Smoothie ES. My current road bike is a Giant Defy 1 (circa 2009). My question is if I did purchase the Soma Smoothie ES how much of my Giant Defy could simply be swapped over to the new frame and what would I need to purchase new? I have a fair amount of bicycle wrenching experience but I have never done anything like this. Again, I am looking at this frame swap as a way to save money so if I am going to need to purchase a bunch of new parts I will instead look at buying a complete or just resist the urge to ride something new haha. Thanks!

Save the most money by not replacing the main part of the bike when it is not necessary. Unless your frame is damaged or otherwise finished, every penny you spend on a replacement is not money well spent.

I'm not going to do the details but where ever there's a part that enters the frame (brake mounting, BB, headset, post) or where there is a length (brake reach, axle width) or gets placed around the frame (ft der, headset/stem) are possible points of difference. Andy.

About like swapping parts from a Toyota to a Honda; they're both cars, and about the same size, right? How do you think that will work out?

+1

New frame might require these parts changed (BB, fork bearings etc). If the frame fits and isn't damaged, I'd never change it. Though it is a good wrenching exercise and could be a fun and learning experience.

N+1 is better. Never have just one bike.

I have swapped components to different frames many, many times. Actually, my bike shops have done it, and it generally costs about $100-150 labor, depending on how much time it takes. I usually end up having to buy a few different components, which adds to the cost. I would check with your bike shop, unless you plan to do the work yourself. If you order the Soma frame through your LBS, they might give you a break on parts and/or labor.

I actually had such a frame swap done with my touring bike last fall, moving parts from my Bob Jackson World Tour to a new Soma Saga frame. I had to buy a new stem, headset and rear wheel but everything else fit fine.

The Toyota/Honda comparison is a bit over blown, but the commonality between the two frames (or lack of it) means you will probably need a frame specific
headset, and maybe a fork, depending on whether the fork steerer tube is long enough for the longer head tube of the Soma. You will need new long reach
brakes and ?? seatpost, as the specs for the Giant frames do not mention seat tube diameter. The bar, brifters, BB and chainwheels, wheels and deraillers all
will swap over (along with the stem perhaps). Then the question arises of what to do with the stripped Giant frame. The Soma frame is really a touring frame.

2009 Giant Defy 1 - BikePedia

You will need, at a minimum:

Brakes, seatpost, headset, fork.

You will likely need a new FD as well.

Would agree with this about 15-20 years back, but today, you can take 2 bikes that look similar, and only have the shifter clamps & RD bolt being the same spec, everything else may be different.

For the OP, start a list/spreadsheet, list every part from your current bike, and it's spec's, then do the same, compare, and you have your answer as to what is needed, then work out if it's cost effective.

Unless you've got a high-end group (Dura Ace, Ultegra, etc) on the Giant, I'd sell it as is to defray the cost of building the Soma with new or NOS parts. If you move "everything" over, you'll still need new cables, possibly chain (different chainstay lengths), brakes, possibly a new FD and seat post if the seat tubes differ. Without knowing how the frames compare in every dimension makes this a possibly expensive guessing game, and your Giant will sell much more easily as a complete bike. When I got my Sam a few years ago, I decided to build it with 9 spd Ultegra, which I was able to find NOS for reasonable $$. My initial build came in under what Rivendell was charging for a complete bike—We won't talk about how much I've spent on my baby since then

I really appreciate everyones responses. Many of you have given me some things to think about and made great points. As I said earlier this is simply an idea am toying with and as I've never done anything like it I wanted some advice. I won't take on the project until fall if I decide to do it. I am not against buying a new complete bicycle (I fully believe in the N+1 rule ha) but not all frames are available as a complete and some completes aren't built with the components/specs I like. Those were a couple reasons in addition to saving money that lead me to the idea of simply swapping frames. I have plenty of research to do and I will be back with more specific questions once I have a better idea of what I want to do. Thanks!

I think everyone is being overly pessimistic. I've swapped frames around many times.
Looks like the crank should work, may have to play with the BB. Brakes should work, depends. Will need a seat post. Looks like the Giant has a tapered fork so a replacement fork and a headset. Giant looks like it has a braze on front derailleur so an adapter clamp should work.
Everything else should swap over.