How easy is it to change a B22 light fitting to an E27 one?
it's usually pretty easy to change the fittings: 


make sure electricity is off (turn off the breaker that powers the light)
double check the electricity is off
loosen the screws that hold it up on the ceiling and keep the wires attached. Remember which wire is connected to the ground (bare/green or yellow green striped wire), live (red, orange or blue wire) and neutral (black wire) connections. If your house is newer then the colors may be different: green/yellow stripe for ground, brown/black or gray for live and blue for neutral
connect the wires as indicated in the manual for the new fitting
attach the new fitting to the ceiling
put in new bulb
turn electricity back on, if the breaker doesn't want to stay on you did something wrong and should inspect what you did wrong before trying to "fix" the breaker.
toggle the switch and bask in your accomplishment.