I believe you have to throw the old ones away and buy new ones for next season.

I wouldn't throw them away. There may not be much nutrition left in a used "Grow Bag", but I dig the old compost into my clay soil - anything to make it less dense.

Or sprinkle a layer into your compost bin, or across your flowerbeds.

It is no longer as good for a growing medium, but it will still add organic matter to your garden soil, which is almost always good!