This conversion is best to do right after your tail motor burns out. You need a new one anyway so might as well upgrade. This would be the most cost effective time but honestly. The second you get frustrated from a tail blow out. You need to convert it. Now there are several ways to do this. First is to use a belted mod.
Although many larger heli’s use this kind of system, it is not a good idea for a 300 size due to its large power strain. you do not have enough mass spinning to efficiently run your rotor. It will work for simple hover and even fast flight but if going inverted your tail and main rotors will fight for power and sap each other. This can usually be seen when you go full collective and then rudder. Your collective (throttle) will noticeable loose power and basically blow out but with out the consequences of a tail blow out.
Brush-less Tail Conversion
A second type is a strict brushless tail conversion. This is a much better idea but the trade of is that it’s twitchy. It takes a lot of power, and is very powerful. It has to compensate then shut of and compensate and shut of all in fractions of a second. It just looks and feels twitchy but its completely possible to get away with successfully if done right.
Variable Pitch Brushless Mod
The best way is to combine the best of both worlds in a variable pitch brushless. On a belt driven system your belt powers your rudder but your servo controls it’s angle. Same principle but instead you use a powerful brushless motor to power your rudder. This is the most efficient way of flying your heli. You also have the ease or repairs compared to a belted system which has many components to damage and strip.