Is it possible to 'retrofit' electric flight control actuators to replace existing hydraulic actuators on aircraft?
Technically it's entirely possible to replace these actuators but it's unlikely they will because the economics don't work out. The weight savings you get will be offset by the costs from design, recertification and installation. It's not as simple as bolting something on. 

When you make a big change like this to an airplane you have to take into account the changes it will bring. The actuator may be a different shape, the loss of weight in one area will change the balance of the airplane and affect its structures in ways that have to be accounted for. The forces the new actuator design will put on structures must be accounted for. 

Electric actuators need electricity, that means you need to generate it and supply it. You'll have to run wires for it, maybe change the engines generators so they supply more juice. You may have to change the Ram Air Turbine so control can be maintained if engine power is lost. The airplane's control systems may have to be modified. Emergency procedures, maintenance manuals must be changed, people have to be trained, etc. Finally, all this must be certified with the FAA, EASA, and other bodies before passengers can be taken, only then can the actuators actually be replaced. 

All this costs a lot of money to do, which is why major retrofits like that are rarely done, the ton of weight they save isn't worth the costs of doing it, even over 10 years of flying.