Old mulch should be replaced with new mulch whenever you notice hints of discoloration or decomposition. This is generally every 4-6 years. You should replenish the preexisting mulch with another 1 inch layer on top every 1-2 years. This fresh topdressing of mulch should be the same color as the current mulch and will make the color pop more and give fresh nutrients to the plants. You don’t usually need to remove the pre-existing mulch when adding new mulch. The reason is because the old mulch will slowly decompose as time goes on.