[Tannaton]

Change it.... if it was the engine oil, you wouldn't think twice...

There are some concerns when changing very old fluid that doing so can dislodge debris, but no-one has ever posted on here where that has happened to them.

You can mitigate the risk of this by "sump dumping" - i.e. remove the drain plug and around 3 litres will come out, do that, refill with new, drive the car a few hundred miles and then repeat. Doing that 4 times will get you around a 94% fluid change. You will notice the difference and your gearbox will be very grateful.