automatic tranimission fluid question

[Aberfoyle]

Hi,
I just want to ask does everybody change the automatic transimission fluid by maintence schedule?
My car is year 2000 toyota corolla,it needs to be changed every 24,000km or every 24 month.
It is quite expensive in dealership,79.99 now,and it was 59.99 in year 2000.
How important is that tranimission fluid?
Any any different if I do it in somewhere else?
My car has 89,000 km now,and it was purchased in year 2000 as new.

Thanks in advance for your suggestion and reply.

George