You will most likely NOT be using the same prop. when you convert to brushless.
The most precise way to convert over is to use the flying weight and style of the plane. Everything in regards to electric power is based on total watts, which then converts to watts per pound of aircraft. Trainer/slow flying models require 70-90 watts per pound, sport/aerobatic models require 110-130 watts per pound, and 150+ is for 3D and High Performance models. The total watts of a power system is calculated by multiplying the voltage input from the battery, times the amp draw of the prop/motor combination.
Example: Let say you have a 3lb aerobatic/sport plane - 3.0lbs x 120watts per pound (middle of the road) = 360 total watts required of the system to acheive the proper performance level. Now that you have a requirement, you can start looking at motors and batteries. Most likely, you will be using a 3 cell 11.1 volt LiPo, unless you have a 4lb. + plane. So, if total watts = voltage from battery x amp draw of motor....... 360 total watts = 11.1volts x amps = 32amps required of the motor in order to acheive the desired performance level. So in this example, you would be looking for a brushless motor that can handle a 3 cell LiPo, with a max. watt output at or above 360watts, and can handle a continuous amp draw of 32amps +
I know.......seems confusing if you have not gone thru this process before
......but this is the most precise way to nail down a power system.