You can refresh used soil by mixing it with "garden soil", which is cheap as fuck. (inversely, you can stretch cheap "garden soil" by mixing it with used soil, lol... either way, it's the cheapest route)
I used MG organic garden soil in my first grow, mixed it with promix because it's too hot and needs to be taken down 50/50. Still use it... mix it with my old FFOF soil for my outdoor vegetable garden, but I don't see why you can't refresh old soil with any kind of charged garden soil. (It's mad cheap too, $5 for 2½ cubic ft bag at walmart)
Of course, I'm of the opinion that MG soil should come with a coupon for free flypaper being that every bag is loaded with insects, so I don't recommend it unless you're prepared to deal with all manner of pests.