I'd hate to see a Target episode on it myself
Like Wok has said, the bacteria is in the filters, don't worry about moving all the water, fish prefer clean water to used water, just make sure the new water has the same parameters (pH, kH, dH), to get this you might just have to let it filter/agitate for 24 hours first to stabalise it. Do some large water changes leading up to the shift so the fish get use to fresh water. If you go for setting up the old tank, I recommend having it ready at least 24 hours before you start as there's always last minute things to do.
The good news is you have plenty of time to get new filters established. It only takes two weeks running a filter in an established tank (with established filter) to make the new filter with beneficial bacteria and thus also become established. I'd just buy a couple of large sponge filters with strong air pumps. Then all you have to do is move the fish and filters and fill up the new (old) tank, and do the same when you return them back to your tank again.