I've had the car for a year now, and I have no idea if the tranny oil has ever been changed. I'd like to change it this summer, but I've heard people say that if you haven't changed it by now (90k) don't change it.

Is this BS?

Total BS. It is never too late to change the fluid. I have solved so many tranny problems from people who have not changed their fluid and brought it in not shifting properly. Tranny fluid is loaded with detergents which are designed to clean the inside of your tranny. If you don't change it, the ending is inevitable.

I agree.
Changing the tranny fluid is allways beneficial to revitalize fluid chemistry,
remove harmful deposits, increase pump pressure, revitalize seals etc.
Don't listen to all the old wives tales out there.

Change your tranny fluid dude!

There are some people who claim running SYNTHETIC in an older trans is bad. Just don't do that. Stick with normal fluid, but don't just CHANGE. Have a professional flush done. That way you get it all.

Is a flush something that wouldn't take a day, or would it be easier just to have the shop do it?

You can't do it without making a huge mess. Take it to a shop. 30 minutes or less with your next oil change. Dropping the pan only drains about 1/3. A flush uses your old fluid, pressure from your trans pump, and injects it into one side of a piston. The piston has new fluid on the other side which is forced back into the trans. Essentially, nearly 100% replacement in a couple of minutes. They can also diagnose your trans pump pressure at the same time

The best possible combination would be to have the flush done, then very soon after, drop the pan and change the filter, then top off again. That'll happen next month for me. My flush last year did wonders for the car, but the old filter concerns me.

Thanks for all the advice!