I just did mine with a big syringe, alot of patience.
It should take just a touch under 1 litre before it reaches the plug.

I definately wouldn't feel like the job was complete if I only 'topped' it up as opposed to getting all the old stuff out first.
To a lesser degree it's like, only ever topping your engine oil up rather than getting a full oil change every 3000/6000 Miles