I pretty much agree with Italiano. Much easier to purchase fresh if you can.
Most or all of the buffering properties in the soil are depleted. Also, you don't want the old roots to start rotting while a new plant is trying to thrive. There may be a nutrient-dump from the old roots composting, too.

However, in emergencies I will remove all old roots, and flush the soil with properly ph'd water. Then, in a five gallon bucket I'll mix the old soil with some fresh garden lime, and a little bone meal. Then, flush well again (in a pot that drains, lol) with properly ph'd water to dissolve the lime into the mix. If you've got a ph pen, you can check your ingoing water ph, and runoff ph. Compare the two...the difference is what your 'replenished' soil is doing to the water. (it's buffering capability)

As with everything in cannabis growing, go easy with the lime and the bone meal. Easier to add more, and once it's in, it's in. There's no taking it back out. Results may vary, and this is no gurantee of success. I am comfortable doing this because I know how to identify and correct any problems that may arise. Newbies generally are not so fortunate, lol.