If you currently have gates, then they can usually be modified to allow automation. However the age and strength of the gate will depend on the amount of modification and the additional cost involved. Some customers find it more economical just to buy new gates.
Automatic gates are built to work with the operator and will be stable throughout use. Gates that have not been made with automation in mind may sway, or be less stable in operation. This can affect the performance and life expectancy of the system.