I use old mix at the beginning of every season. :=) Depending on where I was storing it, I pour it in a disposable pan and bake it in the oven for a bit.

I bought some bags of Miracle Grow potting mix with moisture control on sale at the end of the season last year. I have used it this year with no side effects. My bags were kept outside on the ground. Not many holes in the bags. I brought the bags inside before freezing temps hit. I will be buying the discounted bags again this year. I got the bags (55 qt) for $5.00 last year and the bags this year was $11. Both at Sams. I wished I had picked up more bags last fall, at the very last, for $3/bag.