With traditional plug systems, you have to take down the entire railing and rebuild from scratch.
Make sure to measure the gap between your upper and lower 2×4 rails and pick the 25″ or 31″ and then cut them down to your specific gap less 1/2″ for the thickness of the base connectors.
Aluminum is so easy to cut and is not subject to oxidization the same way ferrous metals are.
This saves you a lot of time and money. But the best thing is that you just gave your old worn out wood railing a brand new life.