Changing To A New Filter

The U2 has a central box containing biomax, two boxes containing a sponge each and two poly-carbon pads.
Your current media will have bacteria on both the sponge and the carbon floss thingy.

Depending on just how big your current sponge is, I would cut it up, using a fluval sponge as a guide, to make it fit in one of the sponge boxes. If it is too big for it all to fit, then put the rest in the other box. Cut up the fluval sponge(s) to fill any space left.
If you can persuade the carbon floss thing to go where the poly carbon pads go in the U2, put them there. If not, maybe scrunch them up and squash them in with the biomax (leave some biomax out if necessary). If you do the later, don't bother with the poly carbon pads that come with the U2. You only need filter wool/floss there, you don't actually need the carbon. I don't know if you can peel these pads apart but if you can, that's what I'd do and just use the white side.
Run the filter like this for a month or two. If you end up putting the carbon floss from the old filter into the biomax chamber, remove that and put the unused biomax in. Wait a month then swap the old sponge for the proper U2 sponge.
Keep an eye on your ammonia and nitrite for a few days to be sure.

Keep the old internal in the cupboard. If you ever need to run carbon, just buy a pack of carbon granules, and a mesh bag (I make them from net fabric) and use the old filter. Just make sure no part of the bag can get into the impeller.

If that's what the carbon thing is like, cut it off the frame and squeeze the floss part in anywhere it'll go. Throw away the carbon. Then in a month or two, the floss stuff should be the first to go. Yes you will lose some bacteria on the carbon inside the cartridge, but hopefully not many. You'll also lose some from the plastic casing of the filter, but again not many. The sponge and floss should contain most of the bacteria, and you'll have some on every other surface in the tank, so they should make up the losses very quickly.