The only time I would remove the old mulch is if it was way too thick and messing up the plants, but it would cost. I did have a new customer last year that wanted some of it removed, because it was piled up high on the plants, I took a rake and moved it away from the plants and they did just fine. It should have broke down, but it was too thick.
I went down to the Crossroads
Fell down on my knees