Cut the sponge so it fits into the new filter. Put it on top or together with the new sponge media so that the bb spreads over to the new media as well.
The bb will eventually take over the new media.
So when you do maintenances on your new filter in future, just take out a section or two of old cut up media every time you do maintenance until you’re eventually left with just the new media that will then be covered in bb.
It might take maybe 3 or 4 maintenance cleans in taking only a few smallish bits of old media to be left with just the new media.
Or just leave them all in the filter, entirely up to you.
Floating the media in the tank does work to a certain extent but unsightly and takes longer for media to establish in the new filter imho.
The bb will eventually take over the new media.
So when you do maintenances on your new filter in future, just take out a section or two of old cut up media every time you do maintenance until you’re eventually left with just the new media that will then be covered in bb.
It might take maybe 3 or 4 maintenance cleans in taking only a few smallish bits of old media to be left with just the new media.
Or just leave them all in the filter, entirely up to you.
Floating the media in the tank does work to a certain extent but unsightly and takes longer for media to establish in the new filter imho.