I'm lately doing some tests with Debian experimental packages thus I often upgrade some packages to experimental and downgrade them back to unstable.
WARNING: Downgrading in Debian is not supported etc.
aptitude search "?narrow(?installed,?archive(experimental))" -F %p|\
sed 's,\([^ ]*\),\1/unstable,'|xargs echo
This will give you a list of experimental packages installed on your system each concatenated with "/unstable". The output can go straight to "aptitude install". I don't directly use "xargs aptitude install" because it's not interactive.