Bicycle Mechanics - new cassette - do I need a new chain?

I currently run a 50/34 ultegra compact with a 10 speed 12-27 rear cassette.

I am just about to buy new wheels, and want to go for a 10 speed 11-27 dura ace cassette upgrade while I'm at it. There won't be any problems as far as derailer settings or chain length etc because of the slightly smaller small cog will there?

As I am buying the wheels and cassette on line, they will be supplied separately. Is it possible to fit the cassette to the new wheels myself, or will it require special tools and a bike mechanic?

Finally, because I am getting a new cassette, will I need a new chain?

I'd appreciate some help.

Thanks in advance.

If they're both Shimano hubs you probably won't need to adjust the derailleur.
Chain stays the same length because you size it to the largest cog.
You will need a lock ring tool and a wrench to tighten it to the freehub. They're not expensive.
http://i154.photobucket.com/albums/s262/toronto505/Decorated%20images/IMG_4531.jpg
You'll need to change the chain if it has stretched. There are tools for guaging this.
Or you can measure it with a good ruler, from one pin center along 12" of chain.
If the 2nd pin is more than 1/16" form the 12" mark on the ruler, you should consider replacing it.

Check out Park Tools website for basic mechanical help. This link is for cassette removal/installation. http://www.parktool.com/repair/byregion.asp?catid=4&imageField2.x=15&imageField2.y=8

I'd change the chain if it has much use on it, but don't throw it away. To maximize cassette life, consider using 3-4 chains in a rotation. Changing chains prematurely, or even at .5% elongation will eventually result in a new chain skipping on the worn cogs. Alternating several chains insures that you will not get chain skip over the life of all 3-4 chains and allows the chains to be used longer.

Measuring elongation alone does not work with all chain brands. A Campy chain can show very little elongation after 5,000 miles of use, but still be extremely worn. The roller spacing still increases and so does the side clearance.

Don't use a chain checker for measuring chain wear. Use a precision 12" rule to measure elongation and calipers to measure roller spacing.

If they're both Shimano hubs you probably won't need to adjust the derailleur.
Chain stays the same length because you size it to the largest cog.
You will need a lock ring tool and a wrench to tighten it to the freehub. They're not expensive.
http://i154.photobucket.com/albums/s262/toronto505/Decorated%20images/IMG_4531.jpg
You'll need to change the chain if it has stretched. There are tools for guaging this.
Or you can measure it with a good ruler, from one pin center along 12" of chain.
If the 2nd pin is more than 1/16" form the 12" mark on the ruler, you should consider replacing it.

Measure the chain with it on the big ring and big cog.

Your riding style affects this, also. I recently put a new chain on my mountain bike; the old one measured 1/16th inch long. It runs just fine on the chainrings and most cassette cogs, but skips in 2, 3, and 4 because those are the ones I use the most when climbing the long dusty hills here. So, I really should have replaced the chain a while back... or just waited and replaced cassette and chain later. This way, though, saves my chainrings. I wish we had better systems that are more dirt-tolerant.

Thanks for the help everyone.

For a new cassette I would go with a new chain. If the old one is in good shape (less than 1/16" in 12") you can reuse it when the new one wears out.