Remake a new coat out of old down coat filling?

I have a 30 year old Eddie Bauer super long down coat that has seen its better days.  Now there are too many rips, tears, patches, and worn areas to keep the down in for another 30 years.  But there's lots of down, and Eddie Bauer down is the cleanest I've ever experienced (no smells).

So, not sure what to do with the old coat yet, I bought a brand new down coat from another good manufacturer for a good price in after season sales.  Unfortunately, I started sneezing up the wazoo when I put it on for 10 minutes after I got home to model it to my husband.  Something was wrong with the down, in my opinion (not cleaned enough?), and so I sadly had to return it.  I have a down vest, another down jacket, and a down comforter I sleep under every night.  All fine.  Alas, I'm getting off topic.

So, I want to save this down out of this old coat, and perhaps sew a new shorter coat and fill it with this really good Eddie Bauer down. Has anyone ever done such a thing?  I have no clue if this is even reasonable, but I'd hate to see this down be left to sit idly in a coat shell that just isn't practical anymore to wear.

Of course it's reasonable! People have been making down coats for a long time :D  
It's not complex, basically just a puff quilt. There are patterns online for those, and I suspect for down fill coats too.  
I want to see pics when you do it!  :D

Any chance of adding a presentable shell on the outside?

I recall that feathers/down require an incredibly tight weave fabric (ticking) to contain the pokey ends. That could be a challenge in repurposing the down.

(Then again, what do I know? I use square knots when sewing on buttons. )

(Then again, what do I know? I use square knots when sewing on buttons. )

said Douglas Alpenstock. . .
Nothing at all wrong with a square knot and a button - it makes a better job than all those shoddy efforts produced by lazy manufacturers!

If you love the down but not the shell, transplanting it into a new shell would be the easiest fix. It wouldn't be impossible to sew your own shell, but a good shell will have baffles between the areas where the down sits, and those are fiddly and annoying to sew correctly. Of course, if you don't care about baffles, then quilting the down between each pair of pattern pieces would be pretty straightforward even by hand or on a domestic sewing machine.

Personally I'd thrift or yard sale a nice shell that's lost its down, then vacuum all the down out of the old one and make the world's fanciest pet bed or something. Give your new shell a good wash while it's empty -- you can use much more heat and stronger soap than you would when it has down in it. Then I'd figure out how to transfer the down from old to new... I'd have to go down the youtube hole and watch other people filling down items to get a clue how to do that at home.

Rule of thumb when opening up something you'll want to reassemble later, like a new shell, is that you only ever cut the stitching threads, never the fabric itself. A seam ripper is the correct tool for this job; tiny scissors also work.

If you're reusing the down into something quilted, I'd strongly recommend considering a flat-lining approach -- cut the lining and outer for each pattern piece, sew them together right-sides-out right along where you'll later put the seams to assemble the garment, put the down in, close the hole you put the down in through, fluff it to distribute the down evenly (like the world's flattest and funniest-shaped pillow), then quilt the whole thing to keep the down in place. Then you'll basically have a bunch of little separate quilts shaped like the pieces of the coat you're trying to make, and there will be down where you need it but no down making the actual seam bulky when you assemble the final thing. If you want to get really fancy you could french seam the edges of each piece before putting the down in, but that's overkill when you could just put bias tape over the raw seam allowances after assembling the final garment, or use a serger if you have one.