SetsulIf it's still a paste and not dried up it's fine. Depending on the paste it can't even dry up.
So you can use to reapply the thermal paste on an old cooler.
A new cooler should come with new thermal paste or even preapplied paste so using that will obviously be better.
that is the case with the Corsair H60 I installed, I just used the reapplied paste it had for now, but I'm having difficulty overclocking my 8700k beyond 4.5Ghz, so I though replacing the current thermal paste with something higher grade would help.