Recently, chris-pl and I installed a K24 on his 1.9L TD Passat. The K24 is quite a bit larger than the minescule T2 (i think that was the model) that was originally installed. In this case, the exhaust manifold flange was the same between the two so we didn't need to change it. The install went fairly smoothly and works quite well. When it left my shop it ran as expected, that is, with a bit more lag off the line (easily negated by the proper launch technique) Boost climbs quite quickly, faster than a 1.6L, which makes sense due to the larger displacement. At that point, no boost controller was installed so it was limited to a non-gated 9 psi. Once Chris puts the boost controller, we'll hear how it goes.
Difficulties to expect when doing such a swap:
You will have to clearance the intake manifold to fit the larger compressor housing. The 1.6L intake manifold has a divot adjacent to the compressor housing for clearance. The 1.9L manifold does not. Get out your grinding tools!
The turbo bearing housing sits in a slightly different location, AND the K24 uses a different oil return line. There are different options, but we used a length of rubber hose (suitable for medium pressure, hot engine oil) and clamped it between the K24's pipe (cut just above the flex section) and the 1.9L's pipe. (cut just below the flex section) The hose was $5/foot available at a local hydraulics shop.
The 1.9L's turbo inlet pipe won't fit the K24. (the pipe between the turbo and the airbox) We used an A2 inlet pipe, cut about 5" off the box end, and spliced in an elbow (stolen from audi 5000 hard and soft boost tubes) which fit nicely into the factory airbox.
The oil supply line needs to be 'tweaked' to meet the bearing housing in it's new location. Take your time, little bends here and there, and it'll fit quite nicely.
The downpipe will sit in a slightly different location, but still works without contacting anything.
The turbo to manifold bolts were quite tight, despite having been apart less than 2 yrs ago. The Bentley's suggestion of extensions and univeral joints won't cut it. We found we could fit a ratchet directly on the socket and work it that way. The two inner bolts are very tight, and perhaps a longer socket would have to be ground shorter.
All in all, I'd rate this job as do-able by anyone with a good set of tools and a good supply of spare hoses and boost tubes. It took about 8 hours over three evenings.