Ah I hadn't noticed your sig change Ira, no wonder I got confused
A Whisper filter has little bags in it doesn't it? Swap the old bag for the new one and put it on the new tank, or, if possible, can they be cut in half and have half old half new in each? I don't have a Whisper filter so not sure what is in them. If they have 2 bags, just swap one around so each filter has one old and one new.