Scion XD began @ 35,000 miles shifting 3rd to 4th gear at 60 mph is this normal? on 2008 Scion xD

Transmission fluid is World Standard synthetic fluid. Fluid became brown at 50,000 miles. Now at 90,000 miles fluid is dark brown and gritty. Still have issue with shifting from 3rd to 4th but now goes to 70 or 80 mph before shifting from 3rd to 4th. Toyota mechanic says this is because of shifting sensor and not transmission. Understand flushing transmission can cause greater issues. Also, thought fluid was to NEVER be changed in transmission.

Asked by for the 2008 Scion xD