Change them whenever. They're time-bombs and can go at any time.
My personal procedure is to take the engine out, and put it on a bench, so I can easily get the cam covers off, and wind the engine over manually with a socket bar, until it's off-cam on all the valves.
If you pull the tensioner when the engine is on-cam (pushing valves open) you might, and probably will, cause the chain to come off the cam sprockets, messing up your timing. One tooth incorrect will cause the piston to contact and bend the valve.
You can do it with the engine still in. I think. I take the engine out because I check valve clearances at the same time. Not that I ever actually change the valve clearances. But if one was seriously off I might look into it.