If you want to go from 32 to 64 bit Cygwin but keep all the packages[1], you might find yourself in a spot where you would like to export the list of cygwin packages and also be able to install cygwin with all these packages again. I will tell you how. Open your Cygwin shell and enter
cygcheck -c -d | sed -e "1,2d" -e 's/ .*\$//' > packagelist
This will simply dump a list of installed packages. To install Cygwin 64 with these packages selected, download setup-x86_64[2] and execute it with the command line parameters
./setup-x86_64 -P `awk 'NR==1{printf \$1}{printf ",%s", \$1}' packagelist`
Note, that "-P" expects a comma separated string with packages. If you have a lot of packages, this list can become too large for the terminal. If thats the case, you need to do some heuristic and split the packagelist file into two or more files. Then run the installer on each of these files.