Transmission fluid, when new is a excellent cleaner. It is chuck full of detergents.
Inside your transmission are paper gaskets, rubber seals, brake pad like friction material, and such.
As a transmission ages, it goes through many hot and cold cycles from use and enviornment. The paper and rubber gets brittle inside the transmission.
Used transmission fluid doesn't clean as well the same as when you wash dishes in a sink. The soap gets dirty and doesn't clean as well.
The problem comes in when you indroduce new, fresh fluid. It cleans away the clutch material and, sometimes, the brittle gaskets and seals inside the transmission. This can cause internal hemroging(Sp?) causing the transmission fluid to go where it isnt needed. Automatic transmissions work on directing fluid, under pressure, to engage gears. Internal fluid leaks bleed off needed pressure and the transmission gives up.
Now that you are scared, let me tell you the good.
The ZF transmissions used in Land Rovers are some of the most stout transmissions I have seen. Usually, I see these types of failures in Domestic products, as they are build "Just good enough".
Smart money is on changing you fluid often on any auto trans... Help keep the fluids detergetnts fresh.
On a Discovery, i would have no problem flushing it.... They are hard core enough to take it.... And, if it kills the trans, frankly, it was on its last leg anyways.