Is it possible to change a gas engine into a electric airplane.

it is possible to change an airplane that was set up to run gas to run on electric instead. you must lighten it if possible to maintain performance. Many gas airplanes are over-strengthened to withstand the loads from a gas engine. the electric engines do not need as much strength around the firewall especially.

yes
using brushless motors.

you'll need to lighten the airframe which on acompleted model may not be easy. probably require recovering.. contrary to advice I am flying a gas model(goldberg Freedom 20 with a cobalt 15 just fine. it would beifit from even more weight reduction but all in all not bad.

Go to E-Zone http://www.ezonemag.com/ then discussions, then sport planes. That forum covers conversions from glow to electric powered.

Be advised that MANY of the conversions highly recommend brushless motors and LiPo batteries, both of which can be expensive.