I have a 2000 Impreza 4dr sedan with 45,000 miles on it. The 'AT Fluid Temp' light came on and I was wondering if this is just an indicator that the AT fluid/filter needs to be changed (never done) or something worse. Is the transmission still under warranty if it is something worse than changing the fluid? Thanks.

No, you're not under warranty any more.

Check the fluid levels, and considering having the system flushed and fluids replaced.