I am not familiar with the hydro rain, but If it is a (Hydraulic Controller and you want to get the Electric new style you will have some issues.
The only way I know to do this is to find all of your hydraulic valves in the yard. I hope you know where they are or have a plan. You will need to replace all of your hydraulic valves with solenoid style valves. You will have to change the controller and then install new cable to run all of the new valves back to the new controller.
When making the connections to the valves from the controller you will need to use silicone filled wire nuts for direct burial and use valve boxes to put the valves in.
This is a pretty heavy type repair bill. Be careful who you use to do this type of repair.
Good Luck with the Project.