How often should I really change transmission fluid?

The "lifetime fluid" thing is marketing. Toyota actually revised their recommendation — they now suggest changing ATF every 60,000 miles for "special operating conditions" which includes stop-and-go driving (so basically everyone). At 90k you're overdue but not in danger territory. Do a drain and fill (NOT a flush), replace about 3.5 quarts with Toyota WS ATF, and you'll be fine.

Did mine at 95k on my 2014 Camry. Smooth as butter after. The "it'll slip if you change it" myth comes from people who waited 200k+ miles and had worn clutch packs that were only gripping because of the friction material suspended in the old fluid. At 90k you're nowhere near that. Just do a simple drain and fill.