Should we change used substrate each cycle?

Almost all substrat can be re-use it really depends on what you used to know what to do to re-use it. Coco coir degrades over time and rurns into coco peat. Coco peat is too dense, holds too much water and not enough oxygen. However, you can get rid of the peat by rinsing it with a strainer and keep the remaining coir and perlite. Some medium like clay pebble or hydroton are more simple to re-use. Soil and living soil should be conditioned with extra amendments, compost and other good stuff. Promix (peat moss) and rockwool cubes would fall under the non reusable for me. hope this help