If you are not using the old filter, and it is still wet, add it to the new tank and run both filters together.
Or use old media (it it was kept wet) in new filter.
The fish will be happy for now, until the toxins build up - 4 - 6 days down the track.
A test kit would be helpful at this stage as you could use it to daily check ammonia, nitrites and nitrates. That way you know when a water change is needed before toxic levels get too high.