Changing null / no data values

Question

I have a raster where null values are stored as -9999. I want to change these to 0.

Googling suggests this can be done in the raster calculator, but e.g. (layer name = -9999) * 0 doesn't work. And indeed other pages suggest that any formula including a null will return a null. I've also tried r.reclass, reclassify by table and by layer, but none are giving me what I need - which is simply to change -9999 values to 0.