You're going to have to pretty much completely empty the tanks to move them or you risk cracking the base- I'd not leave anything more than substrate in there, some people probably wouldn't even do that.
Are the tanks going in each other's place or different places? At least if it's the latter you only have to do one tank at a time. Swapping them over at the same time will be a nightmare scenario.
So first stop is to get down to the fish shop and get some bags (they should be free). Then bag 'em up, empty the tank (keep the filter in a bucket of tank water), and get it moved as quickly as possible. You might find a cool box useful for stashing the bagged fish, depending on how many you have. Try to minimise the time your fish are bagged up. Also try to catch them quicky, and keep them in the dark, to minimise stress. I'd species bag any shoaling fish too.
A good collection of buckets is invaluable in this sort of mission, you always appreciate having them available. For a quid each or whatever, you might as well have 4 or 5 on the go.
How are you going to manage the water aspect? Fill up with new water and condition it? And the pH?
Thing with this kind of task is you imagine it'll take a relatively short space of time, but it inevitably ends up taking hours. If you manage the swap of two tanks in less than 3 hours I'll be impressed!
