Well I feel a smell of misinterpretation.
What risk of overheating? Cold or warm spark plug is only related to the temperature of burning process inside cylinder (more precisely to the temperature at which spark plug operates in engine). The burning temperature is given by construction of engine and fuel mixture ratio. All these things are *not* affected by spark plugs on carburated engines because it has no way to give a feedback from exhaust back to carbs.
Spark plug is able to work reliably only in relatively narrow band of temperatures (let's say 150 deg. C). If spark plug is not warmed up to target temperature it is not able to clean itself of carbon deposits which naturally occurs in cylinder upon combustion. Warm enough surface of insulator disallows it to deposit and if (when engine cold started) it forms deposit it is able to get rid of it. Contrary if the spark plug heated being warmer than intended it tends to wear much faster. In extreme cases insulator can crack or electrode can melt itself and if piece melted down is large enough it can damage the piston. Yet these extreme cases count only for engines which had some part of fuel mixture preparation or engine construction (cams, compression, etc.) heavily modified.
One thing which is not very standardized among manufacturers of spark plugs is warmer - colder rating. With some of them higher number means *colder* while for other it means *warmer*.
Sometimes for some bikes there is one standard setup and recommendation for either warmer or colder environment if original spark plug does not work well.
So if your spark plugs are not cracked, fired to be with whitish layer nor covered in carbon deposit layer I don't think your bike is eligible for spark plug change.