Is it possible to downgrade from Android version 4.0(ICS) to 3.0(Honeycomb) after upgrading to ICS? Can any one help me.
As I already wrote in my comment: A downgrade (as well as an upgrade) can be performed by flashing the corresponding ROM (for references, see other downgrade questions). To find a matching ROM, please see Where can I find stock or custom ROMs for my Android device?. For the process of flashing, you might want to check with the rom-flashing tag, especially the questions dealing with the SGS2.
You can downgrade to previous versions by flashing the Correspondings ROMS. The Downgrade is possible, only if the downgraded version is compatible with your device. HoneyComb is designed for Tablets. its not supported for smartphones. If Samsung GS2 is your devices, then HoneyComb is not supported, rather you can downgrade to pervious GB versions.