I'm doing the same on replacing the Earth box mix. I pitch the top inch or two, dig out the fertilizer then just add enough new potting mix to mix in about halfway down, and fill it to the 'mounded' state it's supposed to start out with.
I'm finding the mix breaks down some over the summer, as I don't grow any veggies then, just leave the old plastic covers over the boxes to keep weeds and stuff out. It sinks down to an inch or two below the rim, and can get a little mouldy-looking so I just get rid of the top layer.
I'm also trying an amendment in all my pots, beds and also in the Earth boxes, alfalfa pellets. So far the peppers in the one I amended look really good! I just mixed the pellets with the potting mix as I was filling the box, maybe 3 cups of pellets to a box. You can get a 50lb. bag of pellets at any feed store for about $16 and so far since September I've only used half a bag. It's supposed to be absolutely magic for daylilies, so the bed out front where my daylilies will be is getting a big dose soon.
