Hi Sergey -
I would definitely flush it before installing new inks. Not worth the risk of gunking it up with incompatible inks. You have one set of refillable carts?
I've got a full set of refillable carts for a 4880 that was used only with piezoflush before my 4880 died. The label is actually checked 4800 though I know they work on the 4880. Maybe they work on 7880 too. Could chips be the only difference? Walker?
I was intending to use the same set of carts with K7 inks, but I didn't get that far. Of course I would have cleaned them thoroughly first. Two sets of carts would be nice though.
If these happen to be compatible, and you are interested you can have them cheap. I was also going to install new dampers before installing the new inks, so I also have 10 new dampers that I know fit both machines. Those are available cheap too.