Bicycle Mechanics - Can interchange Shimano 10 spd sprocket units?

I would like a 14-27 ultegra 10 speed casette. Shimano does not offer one. I am thinking I will purchase the 14-25 and interchange the 21-23-25 sprocket unit with a 21-24-27 sprocket unit for a 12-27. Will this work?

I am concerned for I notice Shimano has 3 different part numbers for a 21-23-25 sprocket unit dependant on the rest of the casette. In this case 12-25, 13-25, 14-25. Why aren't these interchangeable? Or are they?

I would like a 14-27 ultegra 10 speed casette. Shimano does not offer one. I am thinking I will purchase the 14-25 and interchange the 21-23-25 sprocket unit with a 21-24-27 sprocket unit for a 12-27. Will this work?

I am concerned for I notice Shimano has 3 different part numbers for a 21-23-25 sprocket unit dependant on the rest of the casette. In this case 12-25, 13-25, 14-25. Why aren't these interchangeable? Or are they?

I would expect that you can make the 14-27 work just fine. The difference in the 21-23-25 clusters is probably synchronization, and there may be a slight timing issue when shifting between the 21 and the next smaller cog, but in my experience with making custom 9-speed cassettes if synchronization isn't perfect it's hardly noticeable anyway. I say go for it.

Al

I am concerned for I notice Shimano has 3 different part numbers for a 21-23-25 sprocket unit dependant on the rest of the casette. In this case 12-25, 13-25, 14-25. Why aren't these interchangeable? Or are they?
Another possible is the perversity of logistical numbering systems. Sometimes the identical part has a half dozen different numbers.

I did this and have used all year. No problems ...... a great decision. I use all gears and can lock in a comfortable cadence quickly.

Should work fine...

Here's the exploded .pdf file from Shimano...

http://bike.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/CS/EV-CS-6600-2370_v1_m56577569830608752.pdf

I expect the three different part numbers for the 21x27 cluster cover Dura Ace (Ti cogs) Ultegra (steel cogs but assembled on a spider) and 105 (complete flat plate cogs.) Any will work but the higher lines will be lighter and more expensive.