Do you have to flush transmission in order change fluid

It's a more thorough way to change the fluid, and recommended at certain major intervals. There are parts of the transmission and torque converter where fluid will never drain from without a flush.

I would just change it. You risk transmission problems with a flush. If the old fluid is really dirty I would change the fluid, drive it for a while and change again.

my problems is it got no dipstick to check fluid and every garage call say have to flush and goes to Chevy dealer and they only do it flush can it be change without mechanic