The decision comes just 1 day after TMZ reported the theme park was standing firm ... keeping the Confederate flag flying high post-Charlottesville. A park official had told us there's a fundamental difference between the flag they fly and the Confederate Battle flag ... we weren't buying it.

The park's now done a 180, replacing all 6 flags with American flags.

Here's their party line ... "At Six Flags Over Texas we strive every single day to make people happy and to create a fun, thrilling and safe family friendly experience for our guests. We always choose to focus on celebrating the things that unite us versus those that divide us."