How to install cyanogenmod on a Samsung Galaxy Europa (GT-I5500B)
From the devices page, it doesn't seem like your phone is officially supported - http://www.cyanogenmod.com/devices . As an alternative, see http://www.madteam.co/forum/development/%28development%29-cyanogenmod-7-1-%27galaxy5%27-port/
If you do manage to flash a custom ROM, you can change back provided you use the ROMs supplied by http://samfirmware.com - not official, just a repository of original factory ROMs.