You want to do at least two hours drop acclimatising.
I have heard people say hat it takes days for fish to adjust to differing pH and hardness levels and that drip acclimatising is a waste of time, but I lost a few fish from my LFS, in the first day or two after purchase before I started doing it so I, personally, think it's worthwhile; I just feel I ought to put both sides of the argument out there!
As to filters; you really need all the old media in it, if you're moving all the fish, so take the new media out, put your old in and then fill in the gaps with new media. After a month or so, you can start taking the old media back out, if you want to, but no more than a quarter of the total at any one time, and leave at least a fortnight between the removals.