Is it possible to move the developed countries that use 110V 60Hz to 220V 50Hz using same Transmission line cables that they have installed already?

acutely the 110 and 220v isn't an issue in the transmission lines because the transmission have there own voltage and its not the only concern in this case, to sync to network you need three conditions do be matched in the two networks 1- same voltage 2- same phase 3- same frequency

so that's means ill have to change the network generators speed also and all network protections because many things in protection relay on frequency and so,

that's  why if we want to connect two different networks with different frequencies we use  power electronics to make a device that's changes the frequency and its called Inverter.