Use mature media.
If your current filter can support X amount of fish and you take 1/3 of the media, your current filter will quickly replace the lost bacs and the new filter will be cloned ready for you to start stocking slowly begining with approx 1/3 the amount (not necessarily 1/3 the number) and then build up your stock from there.
what species are you keeping? if your planning on keeping fish that get quite big, get them as juveniles and fish in cycle, they wont be creating much waste, so your going to be saving money that way