Copper reducer before hot water heater
Just doing the math for the area A = πr2 


1/2 inch pipe that is A = π(.25)^2  
3/4 inch pipe that is A = π(.375)^2


You can see the difference in area and that changes the amount of volume that will flow. Your water heater has a flow rate based on its piping and therefore it will be the limiting factor. Your house could handle more. 

The only problem I can foresee possibly happening and would be most likely after install is that your 3/4 pipes could have an air gap but this should dissipate over time by running the water slowly in order to fill the line and build pressure.