well, i tried this out myself, and it works.
what i did, was to take one of the sponges out of my allready cycled tank, and put it in a baisin.
then i took the new filter sponge, and sqeezed all of the muck on to the new one. i just played about with it for a while, until the new sponge was fully covered, and then put both of the sponges back.
your old filter still has lots of bacteria on it, and now so does your new one!!
i found that this method was alot more effective than just depositing your filter muck into the tank.
a) much less mess, and
B) it is instant, whereas with the other method, you have to leave the muck in there for a while, before taking it out.
i found that this was very useful, especially when setting up an emergency hospital or quarrentine tank as it is instant and effective.