Swapping over my fish tank... How do I do it?

You can move about 50% of the old tank water over. Also move the filter from the 40 to the 60 and let it run on that tank along with the new filter (uming you purchased a new one) for several weeks. This will allow the beneficial bacteria from the 40 filter to keep your tank from cycling. After both filters run together for a few weeks you can remove the old one.