How does the spacing between the two chainrings of a double crankset compare to that of a triple crankset. I'm curious because I'll be switching from a triple to a double crankset, and I'm wondering if I'll need a new front shifter. Currently I use a Shimano ST-2200 Triple (which has no trim settings).
All Shimano triple shifters will work on double cranksets. Usually the derailleur and shifter are setup the same way for doubles and triples although it will be necessary to release the cable and start all over on the derailleur setup. I recommend the instructions found at http://www.parktool.com.
You will most likely need a new bottom bracket compatible with your crankset and frame (BB shell).
I don't know what you hope to gain. A double crankset is a bit lighter but the conversion is not that easy unless you are a good mechanic.
Is it really worth it to you?