I don't have a Juwel tank, but their instruction manual is available for download. So, I can't add necessarily to the other posters, but it does specify in the manual that the white filter is for mechanical filtration, the black filter is chemical, and there ought to be TWO types of blue sponge. One of them is coarse, the other is fine.
The manual then states if you do the changes as you have been doing to the white and black filters, then "it is sufficient to carefully clean or replace the coarse blue sponges at 6-12 weeks and the fine blue sponges at 3-6 months only".
Finally, the manual states: "Important - Cleaning or replacing the blue filter sponges will affect the bacterial cultures. We recommend therefore that only one of the sponges at a time is cleaned or replaced, never at the same time."
If you have been changing the white and carbon filters as you suggest, it's immaterial what you do with them as far as biofiltration goes. Helpful bacterial cultures haven't necessarily had time to build up on them. However, changing both types of blue sponge at the same time would likely destroy most if not all of your helpful, established bacteria.
Personally, I wouldn't change any of the filter or sponge material at all. I'd only "carefully clean" them, as the manual and other posters have suggested. You could therefore change the white and carbon filters at the same time, as long as you do NOT change or clean both types of blue sponge at the same time. Replacing both blue sponges could have very bad consequences...