Reroasting coffee beans after a couple of years - is it worth it?

Honestly, I would discard them. The flavor is developed by roasting, but once it’s gone, it’s gone. We are talking about volatile aromatic compounds, that likely have degraded or evaporated (yes, it may even happen in mostly airtight containers).

Re-roasting may create some more, but assuming that the original roast was “as intended”, you would likely create unwelcome notes, like bitterness or burnt flavors.

There’s also a good chance that the oils have oxidized, i.e. gone rancid.

There’s nothing wrong with giving it a try, but I wouldn’t expect results worth the effort.

Toss them in the bin

Two years is a heck of a long time for coffee. 6 Years is just beyond good and evil. If you're really really lucky the coffee is going to taste of wood, cardboard and ash. If you're unlucky, you'll get some funky and moldy notes.

Also re-roasting coffee does absolutely no good to the coffee whatsoever, if anything, you'll end up burning the beans and making them taste even more of ash than they already do.

Coffee should be used within several weeks of the roast date. It may last up to a few months if it's darkly roasted coffee which doesn't lose much of its flavour because it doesn't have that much to begin with.

If you have a garden and don't want to just throw them away, use them as compost!

If you want to see what stale coffee tastes like, by all means, go ahead and try making an espresso with it. You shouldn't really have any negative health effects from one cup if it was stored in an airtight container - James Hoffmann drank coffee from the seventies!